LINKAGE AND CROSSING-OVER SMG
A brief description of Linkage - Bateson and Punnett's Experiment on Sweet pea, Lathyrus odoratus, Coupling and Repulsion Theory, Complete and Incomplete Linkage, Significance of Linkage, Crossing-over: Cytological basis, Types, Factors influencing the frequency , Significance, Mitotic crossing-over
This PPT consists of 15 slides only explaining Pleiotropy. This is a phenomenon when one gene controls more than one trait , the traits may be related .Generally one gene's product acts for many reactions and so can affect more than one trait. Examples can be seen in pea Coloured flower and pigmentation in leaf axil, frizzle trait in chicken, fur colour and deafness in cats,Human pleiotropic traits are PKU,Sickle cell Anaemia. HOsyndrome , p53 gene etc
Basics of Undergraduate/university fellows
In supplementary gene action, the dominant allele of one gene is essential for the
development of the concerned phenotype, while the other gene modifies the expression of the first gene.
Maternal effects are the influences of a mothers genotype on the phenotype of her offspring. It results from the asymmetric contribution of the female parent to the development of zygotes.
In terms of chromosomal genes, both male and female parents contribute equally to the zygote. The female parent contributes to the zygotes initial cytoplasm and organelles. Sperm rarely contribute anything other than chromosomes. Therefore zygotic development begins within a maternal medium and hence the maternal cytoplasm directly affects zygotic development.
GENETICS
CYTOGENETICS
Definition of Linkage, Coupling and Repulsion hypothesis, Linkage group- Drosophila, maize and man, Types of linkage-complete linkage and incomplete linkage, Factors affecting linkage- distance between genes, age, temperature, radiation, sex, chemicals and nutrition, Significance of linkage.
The tendency of two or more genes to stay together (i.e., the co-existence of two or more genes) in the same chromosome during inheritance is known as LINKAGE. The linked genes are present on the same chromosome are said to be SYNTENIC. The linked genes do not show independent assortment.
LINKAGE v/s INDEPENDENT ASSORTMENT
The frequency of linkage or the strength recombination is influenced by several factors (agents).
This PPT consists of 15 slides only explaining Pleiotropy. This is a phenomenon when one gene controls more than one trait , the traits may be related .Generally one gene's product acts for many reactions and so can affect more than one trait. Examples can be seen in pea Coloured flower and pigmentation in leaf axil, frizzle trait in chicken, fur colour and deafness in cats,Human pleiotropic traits are PKU,Sickle cell Anaemia. HOsyndrome , p53 gene etc
Basics of Undergraduate/university fellows
In supplementary gene action, the dominant allele of one gene is essential for the
development of the concerned phenotype, while the other gene modifies the expression of the first gene.
Maternal effects are the influences of a mothers genotype on the phenotype of her offspring. It results from the asymmetric contribution of the female parent to the development of zygotes.
In terms of chromosomal genes, both male and female parents contribute equally to the zygote. The female parent contributes to the zygotes initial cytoplasm and organelles. Sperm rarely contribute anything other than chromosomes. Therefore zygotic development begins within a maternal medium and hence the maternal cytoplasm directly affects zygotic development.
GENETICS
CYTOGENETICS
Definition of Linkage, Coupling and Repulsion hypothesis, Linkage group- Drosophila, maize and man, Types of linkage-complete linkage and incomplete linkage, Factors affecting linkage- distance between genes, age, temperature, radiation, sex, chemicals and nutrition, Significance of linkage.
The tendency of two or more genes to stay together (i.e., the co-existence of two or more genes) in the same chromosome during inheritance is known as LINKAGE. The linked genes are present on the same chromosome are said to be SYNTENIC. The linked genes do not show independent assortment.
LINKAGE v/s INDEPENDENT ASSORTMENT
The frequency of linkage or the strength recombination is influenced by several factors (agents).
A complementation test (sometimes called a "cis-trans" test) can be used to test whether the mutations in two strains are in different genes. By taking an example of Benzer's work, complementation has been explained.
Genetic linkage is the tendency of DNA sequences that are close together on a chromosome to be inherited together during the meiosis phase of sexual reproduction.
Cytoplasmic inheritance and extra chromosomal inheritanceJs Mn
the cytoplasmic inheritance is in which cytoplasm contain self replicating hereditary material of cytoplasm formed of DNA and this DNA govern many specific characters in plants and animals.
A complementation test (sometimes called a "cis-trans" test) can be used to test whether the mutations in two strains are in different genes. By taking an example of Benzer's work, complementation has been explained.
Genetic linkage is the tendency of DNA sequences that are close together on a chromosome to be inherited together during the meiosis phase of sexual reproduction.
Cytoplasmic inheritance and extra chromosomal inheritanceJs Mn
the cytoplasmic inheritance is in which cytoplasm contain self replicating hereditary material of cytoplasm formed of DNA and this DNA govern many specific characters in plants and animals.
Allelic and Non-allelic interactions : Complete dominance; Incomplete dominance-in Four O'clock plant, Mirabilis jalapa and Snapdragon, Antirrhinum majus ; Co-dominance- MN blood group, AB blood group, Roan coat colour in shorthorn breed of cattle; Inheritance of Comb pattern in Poultry; Epistasis -Dominant - Fruit colour in Summer squash, Recessive - Coat colour in mice; Complementary gene interaction -Purple flower colour in Sweet pea (Lathyrus odoratus)
This PowerPoint presentation intends to explore the thought and development of genetics after G.J.Mendel along with the factor hypothesis in this new line of research during the contemporary.
Genetics- Chapter 5 - Principles of inheritance and variation.docxAjay Kumar Gautam
Genetics is a branch of biology concerned with the study of genes, genetic variation, and heredity in organisms. Though heredity had been observed for millennia, Gregor Mendel, Moravian scientist and Augustinian friar working in the 19th century in Brno, was the first to study genetics scientifically. Mendel studied "trait inheritance", patterns in the way traits are handed down from parents to offspring over time. He observed that organisms (pea plants) inherit traits by way of discrete "units of inheritance". This term, still used today, is a somewhat ambiguous definition of what is referred to as a gene.
According to Mendel, the selfing of a dihybrid (Aa.Bb) will result .pdfarjuntiwari586
According to Mendel, the selfing of a dihybrid (A/a.B/b) will result in four groups of
phenotypically different offspring (9/16 dominant-dominant; 3/15 dominant-recessive; 3/16
recessive-dominant; 1/16 recessive-recessive for the two aits involved). In additional to
experimental errors, please list 2 different reasons at make one unable to observe the 9:3:3:1
ratio.
Solution
(1) For each of the seven pairs of characters examined, it was observed that one allelomorph
dominated over the other, so that F1 exhibits one or the other alternative phenotypes represented
in the parents. However, soon after rediscovery of Mendel\'s laws, experiments were available to
show tha tin some cases the F1 individual showed the phenotype which was intermediate
between the two parents. For instance in four-o\'clock plant (Mirabilis jalapa) it was found that
when plants wiht red flowers were crossed with those having white flowers, plants with pint
flowers were obtained in F1 generation. This would then give rise to red, pink and white
flowered plants in 1:2:1 ratio in the F2 generation. Similarly in Snapdragon, plants with broad
leaves and plants with narrow leaves give reise to plants with intermediate leaves in F1
generation. Likewise, plants plants with red flowers and plants with white flowers give rise to
those with pink flowers. If a dihybrid cross is made using broad leaves and red flowers in one
parent (BBRR) and narrow leaves and white flowers in the other parent (bbrr), F1 individuals
(BbRr) will have intermediate leaves with pink flowers. In the F2 generation nine phenotypes
corresponding to nine genotypes will be observed.
(2) According to the Law of Independent Assortment, any two or more than two pairs of
characters assort independently of each other. Exception to this phenomenon were discovered
due to linkage and the associated phenomenon of crossing over.
(3) In each of the seven pairs of characters studied by Mendel, there were only tow laternative
forms for each character. This meant that only two alleles were present for each character. This
also led to a belief that for each character there were two alternative forms, one dominant over
the other. Later work showed that for a character there can be several phenotypes e.g. for rabbits
the body can be of four or more types. Therefore, concept of alternative allelomorphs had to be
modified by the concept of multiple allelism..
NCERT Books Class 12 Biology Chapter 5 Principles of InheritanceExplore Brain
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Principles of Inheritance
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This pdf comprises of Basic of Genetics: Purpose: To convey that “Genetics is to biology what Newton’s
laws are to Physical Sciences”. Mendel’s laws, Concept of segregation and
independent assortment. Concept of allele. Gene mapping, Gene
interaction, Epistasis. Meiosis and Mitosis be taught as a part of
genetics. Emphasis to be give not to the mechanics of cell division nor the
phases but how genetic material passes from parent to offspring. Concepts
of recessiveness and dominance. Concept of mapping of phenotype to
genes. Discuss about the single gene disorders in humans. Discuss the
concept of complementation using human genetics.
Biostatistics Collection of Data and Sampling Techniques SMG.pptxsajigeorge64
Biostatistics : A brief description of collection of data and sampling techniques - Methods of collection of primary and secondary data - census method- sampling methods- merits and demerits of sampling.
A general account of Quantitative (Multiple factor or Polygenic) Inheritance; Examples : Kernel colour in Wheat, Ear size (Cob length ) in Maize(Zea mays) ; Differences between Qualitative and Quantitative Inheritance
Introduction to Genetics - Mendelism SMGsajigeorge64
Introduction to Genetics - Mendelism ; Genetics defenition- heridity and variation - heritable and non-heritable variations; Gregor Johann Mendel - rediscovery of Mendelism- Terminology and symbols; Mendel's experiments , laws
A General account of Plant Peroxisomes - Ultrastructure, Types :Leaf peroxisomes (Leaf -type peroxisomes), Peroxisomes for other special metabolism, unspecialized peroxisomes and Glyoxysomes ; Functions
Changes In Number And Structure Of Chromosomes SMGsajigeorge64
A brief account of the changes in number and structure of chromosomes : Haploidy, Polyploidy, Aneuploidy, Deletion, Duplication, Inversion and Translocation
A general account of special types of chromosomes - Giant chromosomes (Polytene chromosomes or salivary gland chromosomes and Lampbrush chromosomes ) & B chromosomes
The French Revolution, which began in 1789, was a period of radical social and political upheaval in France. It marked the decline of absolute monarchies, the rise of secular and democratic republics, and the eventual rise of Napoleon Bonaparte. This revolutionary period is crucial in understanding the transition from feudalism to modernity in Europe.
For more information, visit-www.vavaclasses.com
A Strategic Approach: GenAI in EducationPeter Windle
Artificial Intelligence (AI) technologies such as Generative AI, Image Generators and Large Language Models have had a dramatic impact on teaching, learning and assessment over the past 18 months. The most immediate threat AI posed was to Academic Integrity with Higher Education Institutes (HEIs) focusing their efforts on combating the use of GenAI in assessment. Guidelines were developed for staff and students, policies put in place too. Innovative educators have forged paths in the use of Generative AI for teaching, learning and assessments leading to pockets of transformation springing up across HEIs, often with little or no top-down guidance, support or direction.
This Gasta posits a strategic approach to integrating AI into HEIs to prepare staff, students and the curriculum for an evolving world and workplace. We will highlight the advantages of working with these technologies beyond the realm of teaching, learning and assessment by considering prompt engineering skills, industry impact, curriculum changes, and the need for staff upskilling. In contrast, not engaging strategically with Generative AI poses risks, including falling behind peers, missed opportunities and failing to ensure our graduates remain employable. The rapid evolution of AI technologies necessitates a proactive and strategic approach if we are to remain relevant.
Biological screening of herbal drugs: Introduction and Need for
Phyto-Pharmacological Screening, New Strategies for evaluating
Natural Products, In vitro evaluation techniques for Antioxidants, Antimicrobial and Anticancer drugs. In vivo evaluation techniques
for Anti-inflammatory, Antiulcer, Anticancer, Wound healing, Antidiabetic, Hepatoprotective, Cardio protective, Diuretics and
Antifertility, Toxicity studies as per OECD guidelines
Instructions for Submissions thorugh G- Classroom.pptxJheel Barad
This presentation provides a briefing on how to upload submissions and documents in Google Classroom. It was prepared as part of an orientation for new Sainik School in-service teacher trainees. As a training officer, my goal is to ensure that you are comfortable and proficient with this essential tool for managing assignments and fostering student engagement.
June 3, 2024 Anti-Semitism Letter Sent to MIT President Kornbluth and MIT Cor...Levi Shapiro
Letter from the Congress of the United States regarding Anti-Semitism sent June 3rd to MIT President Sally Kornbluth, MIT Corp Chair, Mark Gorenberg
Dear Dr. Kornbluth and Mr. Gorenberg,
The US House of Representatives is deeply concerned by ongoing and pervasive acts of antisemitic
harassment and intimidation at the Massachusetts Institute of Technology (MIT). Failing to act decisively to ensure a safe learning environment for all students would be a grave dereliction of your responsibilities as President of MIT and Chair of the MIT Corporation.
This Congress will not stand idly by and allow an environment hostile to Jewish students to persist. The House believes that your institution is in violation of Title VI of the Civil Rights Act, and the inability or
unwillingness to rectify this violation through action requires accountability.
Postsecondary education is a unique opportunity for students to learn and have their ideas and beliefs challenged. However, universities receiving hundreds of millions of federal funds annually have denied
students that opportunity and have been hijacked to become venues for the promotion of terrorism, antisemitic harassment and intimidation, unlawful encampments, and in some cases, assaults and riots.
The House of Representatives will not countenance the use of federal funds to indoctrinate students into hateful, antisemitic, anti-American supporters of terrorism. Investigations into campus antisemitism by the Committee on Education and the Workforce and the Committee on Ways and Means have been expanded into a Congress-wide probe across all relevant jurisdictions to address this national crisis. The undersigned Committees will conduct oversight into the use of federal funds at MIT and its learning environment under authorities granted to each Committee.
• The Committee on Education and the Workforce has been investigating your institution since December 7, 2023. The Committee has broad jurisdiction over postsecondary education, including its compliance with Title VI of the Civil Rights Act, campus safety concerns over disruptions to the learning environment, and the awarding of federal student aid under the Higher Education Act.
• The Committee on Oversight and Accountability is investigating the sources of funding and other support flowing to groups espousing pro-Hamas propaganda and engaged in antisemitic harassment and intimidation of students. The Committee on Oversight and Accountability is the principal oversight committee of the US House of Representatives and has broad authority to investigate “any matter” at “any time” under House Rule X.
• The Committee on Ways and Means has been investigating several universities since November 15, 2023, when the Committee held a hearing entitled From Ivory Towers to Dark Corners: Investigating the Nexus Between Antisemitism, Tax-Exempt Universities, and Terror Financing. The Committee followed the hearing with letters to those institutions on January 10, 202
Read| The latest issue of The Challenger is here! We are thrilled to announce that our school paper has qualified for the NATIONAL SCHOOLS PRESS CONFERENCE (NSPC) 2024. Thank you for your unwavering support and trust. Dive into the stories that made us stand out!
2024.06.01 Introducing a competency framework for languag learning materials ...Sandy Millin
http://sandymillin.wordpress.com/iateflwebinar2024
Published classroom materials form the basis of syllabuses, drive teacher professional development, and have a potentially huge influence on learners, teachers and education systems. All teachers also create their own materials, whether a few sentences on a blackboard, a highly-structured fully-realised online course, or anything in between. Despite this, the knowledge and skills needed to create effective language learning materials are rarely part of teacher training, and are mostly learnt by trial and error.
Knowledge and skills frameworks, generally called competency frameworks, for ELT teachers, trainers and managers have existed for a few years now. However, until I created one for my MA dissertation, there wasn’t one drawing together what we need to know and do to be able to effectively produce language learning materials.
This webinar will introduce you to my framework, highlighting the key competencies I identified from my research. It will also show how anybody involved in language teaching (any language, not just English!), teacher training, managing schools or developing language learning materials can benefit from using the framework.
The Roman Empire A Historical Colossus.pdfkaushalkr1407
The Roman Empire, a vast and enduring power, stands as one of history's most remarkable civilizations, leaving an indelible imprint on the world. It emerged from the Roman Republic, transitioning into an imperial powerhouse under the leadership of Augustus Caesar in 27 BCE. This transformation marked the beginning of an era defined by unprecedented territorial expansion, architectural marvels, and profound cultural influence.
The empire's roots lie in the city of Rome, founded, according to legend, by Romulus in 753 BCE. Over centuries, Rome evolved from a small settlement to a formidable republic, characterized by a complex political system with elected officials and checks on power. However, internal strife, class conflicts, and military ambitions paved the way for the end of the Republic. Julius Caesar’s dictatorship and subsequent assassination in 44 BCE created a power vacuum, leading to a civil war. Octavian, later Augustus, emerged victorious, heralding the Roman Empire’s birth.
Under Augustus, the empire experienced the Pax Romana, a 200-year period of relative peace and stability. Augustus reformed the military, established efficient administrative systems, and initiated grand construction projects. The empire's borders expanded, encompassing territories from Britain to Egypt and from Spain to the Euphrates. Roman legions, renowned for their discipline and engineering prowess, secured and maintained these vast territories, building roads, fortifications, and cities that facilitated control and integration.
The Roman Empire’s society was hierarchical, with a rigid class system. At the top were the patricians, wealthy elites who held significant political power. Below them were the plebeians, free citizens with limited political influence, and the vast numbers of slaves who formed the backbone of the economy. The family unit was central, governed by the paterfamilias, the male head who held absolute authority.
Culturally, the Romans were eclectic, absorbing and adapting elements from the civilizations they encountered, particularly the Greeks. Roman art, literature, and philosophy reflected this synthesis, creating a rich cultural tapestry. Latin, the Roman language, became the lingua franca of the Western world, influencing numerous modern languages.
Roman architecture and engineering achievements were monumental. They perfected the arch, vault, and dome, constructing enduring structures like the Colosseum, Pantheon, and aqueducts. These engineering marvels not only showcased Roman ingenuity but also served practical purposes, from public entertainment to water supply.
Operation “Blue Star” is the only event in the history of Independent India where the state went into war with its own people. Even after about 40 years it is not clear if it was culmination of states anger over people of the region, a political game of power or start of dictatorial chapter in the democratic setup.
The people of Punjab felt alienated from main stream due to denial of their just demands during a long democratic struggle since independence. As it happen all over the word, it led to militant struggle with great loss of lives of military, police and civilian personnel. Killing of Indira Gandhi and massacre of innocent Sikhs in Delhi and other India cities was also associated with this movement.
Macroeconomics- Movie Location
This will be used as part of your Personal Professional Portfolio once graded.
Objective:
Prepare a presentation or a paper using research, basic comparative analysis, data organization and application of economic information. You will make an informed assessment of an economic climate outside of the United States to accomplish an entertainment industry objective.
BÀI TẬP BỔ TRỢ TIẾNG ANH GLOBAL SUCCESS LỚP 3 - CẢ NĂM (CÓ FILE NGHE VÀ ĐÁP Á...
Linkage and Crossing-over.pptx
1. LINKAGE AND CROSSING-OVER
Dr Saji Mariam George
Associate Professor (Retired)
Assumption College Autonomous
Changanacherry
2. LINKAGE AND CROSSING-OVER
• Genes (segment of genetic
material – DNA in most organisms
and RNA in some viruses) or
hereditary units that control a
particular character are located
in chromosomes. In other words,
chromosomes are the physical
bearers of hereditary units called
genes(Chromosome theory of
heredity- proposed independently
by Walter Sutton - based on
studies in Grasshoppers and
Theodor Boveri - based on studies
in Sea urchins, 1902 - 1903).
Walter Sutton Theodor Boveri
https://biology-igcse.weebly.com
4. • Let ‘P’ stands for the allele for purple flower colour and ‘L’ , for the
allele for long pollen grains. So, the genotype (genetic constitution)
of the pure breeding homozygous Sweet pea parent plant with
purple flowers and long pollen grains can be represented as PPLL .
• From previous hybridization experiments in Sweet pea, it had been
found that, purple flower colour is dominant over red and long
pollen grain is dominant over round. Hence the genotype of the
other Sweet pea plant with red flowers and round pollen grains
(both recessive traits) can be represented as ppll .
• All the F1 (First filial generation) plants had purple flowers and long
pollen grains (hybrids, Genotype in heterozygous condition, PpLl) as
expected.
• On further crossing (selfing, F1 x F1 – PpLl x PpLl ), the F1 plants
produced the F2 generation (Second filial generation).
[Alleles : Two or more alternative forms of the same gene having different
phenotypic effects]
5. • Bateson, Saunders and Punnett had found that the F2
phenotypic ratio deviated significantly from the expected
typical dihybrid F2 phenotypic ratio, 9:3:3:1 ( That is, 9 purple
long : 3 purple round : 3 red long : 1 red round - This ratio is
obtained when there is independent assortment of two
genes that control the two traits).
• They had found that two parental classes (purple flowers with
long pollen grains and red flowers with round pollen grains)
were formed in large numbers (over represented) and the
non-parentals (i.e., recombinants - purple flowers with round
pollen grains and red flowers with long pollen grains) were
formed only in lesser numbers (under represented) than
expected.
6. Image :https://www.chegg.com
F2 : Purple long(parental type, formed in more numbers ) , purple round (non-
parental type or recombinants , formed in less numbers ) , red long (non-parental
type or recombinants, formed in less numbers ) , red round (parental type, formed
in more numbers than expected ). F2 phenotypic ratio deviated significantly from
the expected 9:3:3:1 – Indicates lack of independent assortment between the two
genes that control the two traits , flower colour and shape of pollen grains.
7. • A test cross between F1 dihybrid and the homozygous
double recessive parent (PpLl x ppll) also showed deviation
from the expected dihybrid test cross ratio 1:1:1:1 (i.e., 1
purple long : 1 purple round : 1 red long : 1 red round . That
is, both parental types of progeny and the non-parental
types or recombinants are formed in equal proportions).
Instead, the test cross progeny included more number of
parental types (purple long and red round) and less number
of non-parental types or recombinants (purple round and
red long).
• Bateson, Saunders and Punnett had analysed their
observations and proposed the ‘Coupling and Repulsion
Theory’ to explain the lack of independent assortment .
[ Test cross: A cross between F1 hybrid and the homozygous recessive
parent)
8. Coupling and Repulsion Theory
Coupling Phase
• According to Bateson, Saunders and Punnett , in Sweet pea,
there might be a connection between the parental alleles for
flower color and shape of pollen grains which resulted in
lack of independent assortment and deviation from the
expected dihybrid F2 phenotypic ratio, 9:3:3:1. That means,
the genes for flower colour and pollen shape tend to
remain together and do not assort independently as per
Mendel’s law of independent assortment.
9. • Coupling phase can be defined as the condition in which the
two dominant alleles tend to enter the gametes together in
greater than random proportion. The Sweet pea double
heterozygote (dihybrid) PpLl (Purple long) in coupling phase has
two dominant alleles , P and L from one parent (Genotype PPLL
– Sweet pea with purple flowers and long pollen grains) and
two recessive alleles, p and l from the other parent (Genotype
ppll- Sweet pea with red flowers and round pollen grains) and
they tend to enter the same gamete and to be transmitted
together and hence there is no independent assortment .
10. The double heterozygote , PpLl in
Coupling Phase
(PPLL x ppll → PpLl – Coupling double heterozygote)
Here, the dominant alleles, P and L are on one chromosome and their recessive
alleles p and l are on the other chromosome of a homologous pair – i.e., in
Coupling phase.
[Homologous pair : The maternal and paternal chromosomes]
11. • The genotype of the F1 double heterozygote, PpLl can be
written as PL/pl to denote the coupling phase, where the
slash (/) separates alleles inherited from different parents. This
means that the alleles on the left and right of the slash are on
different homologous chromosomes, one from each parent .
• The coupling might have prevented their independent
assortment in the F1. Hence the parental combinations of
alleles P (dominant allele for purple flowers) and L ( dominant
allele for long pollen grains) and p (recessive allele for red
flowers) and l (recessive allele for round pollen grains) were
more prevalent than the non-parental combinations P and l and
p and L in the gametes of the F1 plants.
12. • Self fertilization of F1 therefore, produced more number of
plants with parental types (purple flowers with long pollen
grains and red flowers with round pollen grains) and less
number of non-parental types or recombinants (purple
flowers with round pollen grains and red flowers with long
pollen grains ) in the F2 generation than expected.
• The non-parental types of progeny or recombinants have
one character from one parent and another character from
the other parent. That is, in purple round types, the trait
purple flower colour from one parent and the round pollen
grain shape from the other parent. Similarly, in red long
types, the trait red flower colour from one parent and the
long pollen grain shape from the other parent.
13. Repulsion Phase
• Repulsion denotes the phase where the double heterozygote ,
PpLl has two dominant alleles P and L and recessive alleles p and
l from two different parents. That is, from a cross between a
Sweet pea plant with purple flowers and round pollen grains
(Genotype PPll ) and another one with red flowers and long
pollen grains (Genotype ppLL) and they tend to enter different
gametes and to remain apart. This is called gametic repulsion.
• In this case, the dominant allele P and the recessive allele l are
found on the same chromosome and the dominant allele L and
the recessive allele p are found on the other chromosome of a
homologous pair. In other words, the non-allelic dominant alleles
“repelled” each other – This is called repulsion. That means, each
homologous chromosome has one dominant allele and one
recessive allele of the two genes. That is, there is a tendency for
one dominant allele of one trait and one recessive allele of
another trait to enter the gametes in greater proportion.
14. The double heterozygote , PpLl in
Repulsion Phase
(PPll x ppLL → PpLl – Repulsion double heterozygote)
Here, dominant allele of one gene and the recessive allele of
the other gene are found on each homologous chromosome .
i.e., in Repulsion phase.
15. • The genotype of the double heterozygote,
PpLl in Repulsion phase can be denoted
as Pl/pL which means that two dominant
alleles P and L or recessive alleles p and l
are introduced into the double
heterozygote by two different parents
and hence are located in two different
chromosomes of a homologous pair and
are inherited separately.
• Thus, the double heterozygote PpLl may
be either in coupling phase (PL/pl) or in
repulsion phase (Pl/pL).
Haldane (British
scientist, 1942)used
the term Cis for
coupling phase and
Trans for repulsion
phase.
J.B.S Haldane
(John Burdon
Sanderson Haldane)
16. • Even though Bateson, Saunders and Punnett had tried to
explain their results in Sweet pea breeding experiments
involving two pairs of contrasting characters (Flower colour :
Purple vs. Red ; Shape of Pollen grains : Long vs. Round) by
proposing the Coupling and Repulsion Theory, they had
failed to give a precise explanation for the deviation from
the typical Mendelian ratios (i.e., dihybrid F2 phenotypic
ratio, 9:3:3:1 and dihybrid test cross ratio, 1:1:1:1 )and lack
of independent assortment.
• Later, the Coupling and Repulsion Theory was replaced by
the Theory of Linkage and Crossing-over put forward by
Morgan et.al., (1911).
17. Morgan’s Theory of Linkage and Crossing-over
• Thomas Hunt Morgan (1910) and
his associates (William Ernest
Castle, Alfred Henry Sturtevant,
Hermann Joseph Muller and Calvin
Blackman Bridges, Columbia
University, New York) had carried
out breeding experiments in fruit
fly (vinegar fly), Drosophila
melanogaster.
• Eventhough , it was Sutton and
Boveri (1902-1903) who proposed
the chromosome theory of
heredity, the first experimental
evidence for it was given by
Morgan et. al., – the studies on
inheritance of white-eyed trait in
Drosophila.
T. H. Morgan W. E. Castle
A. H. Sturtevant
H. J. Muller
C. B. Bridges
18. • Morgan et. al., had found that Coupling and Repulsion are
two aspects of the same phenomenon called Linkage. They
had proposed that two genes are found in coupling phase
because they are present on same chromosome and are
said to be linked.
• Similarly , two genes are in repulsion phase because they are
present on two different homologous chromosomes.
19. Chromosome Theory of Linkage
Morgan and Castle (1911) had proposed the Chromosome Theory of
Linkage based on studies in Drosophila. The important postulates of
the theory are
The genes are linearly arranged in a chromosome. Each gene is
located at a specific point called locus. (Plural: Loci).
Genes located in the same chromosome tend to remain together and
are inherited as a single unit. This is known as linkage. In other
words, linkage can be defined as the tendency of the genes present
in the same chromosome to be inherited together as a single unit
during meiosis. All the genes located in a single chromosome
constitute a linkage group. The total number of linkage groups in an
organism corresponds to the haploid chromosome number (or equal
to the number of homologous chromosome pairs ).
Linked genes tend to stay in parental combinations.
20. The strength of the linkage depends upon the distance
between linked genes in a chromosome. Genes that are
located very close to each other show stronger linkage than
the genes that are far apart from each other. That is, the
strength of linkage is inversely proportional to the distance
between linked genes.
21. Linkage Groups – Number of linkage groups is same as that of the
haploid chromosome number of a species : Examples
Organism Haploid
Chromosome
Number
Linkage Groups
Sweet Pea (Lathyrus odoratus) 7 7
Maize (Zea mays) 10 10
Fruit fly or Vinegar fly
(Drosophila melanogaster)
4 4
Onion (Allium cepa) 8 8
22. Types of Linkage
• There are two types of linkage viz.
complete and incomplete .
1. Complete Linkage
• The genes that are very closely
located in a chromosome show
complete linkage and are inherited
together as a single unit. Hence
only parental types are produced
in succeeding generations.
• There is no independent
assortment and crossing over.
Hence no recombinants are
produced. However, complete
linkage is very rare - reported only
in male Drosophila melanogaster
and female silk moth, Bombyx
mori .
Drosophila melanogaster male fly
https://www.semanticscholar.org
Female silk moth Saved from aureus-butterflies.de
Saved by Carmen Gómez
https://www.pinterest.com
Female silk moth, Bombyx mori
23. Example : Complete Linkage in male Drosophila melanogaster
• Morgan and Lynch (1911) had found that in Drosophila, the
genes for body colour (gray vs. black) and wing size (long vs.
vestigial ) were linked to each other.
• It is also to be noted that the allele for gray body colour is
dominant over black and the allele for long wings is dominant
over vestigial wings.
Gray body, long wings Black body , vestigial wings
https://sciencemusicvideos.com
[Long wings : Normal functional wings
Vestigial wings : Shriveled or crumpled, non-functional wings ]
24. • Let ‘G’ represent the allele for gray body colour and ‘ g’ for
black body colour.
• Similarly, let ‘L’ represent the allele for long wings and ‘l’ for
vestigial wings .
• Let us consider a cross between a male Drosophila fly with
gray body and long wings (Genotype GGLL) and a female fly
with black body and vestigial wings ( Genotype ggll).
• Since the gene for gray body colour is dominant over black
and the gene for long wings is dominant over vestigial
wings, all the F1 flies have gray body and long wings
(Genotype GgLl).
25. • A test cross between a F1 male fly with gray body and long
wings (Genotype GgLl , can be represented as GL/gl in Coupling
or Cis linkage phase) and the double recessive female parent fly
with black body and vestigial wings (Genotype ggll) , produced
only parental types of progeny (i.e., with gray body, long wings
and black body , vestigial wings) in a 1 : 1 ratio(That means,
50% of the progeny with gray body, long wings and 50 % with
black body , vestigial wings) with no recombinants instead of
the normal dihybrid test cross ratio 1: 1: 1: 1. (That is, 1 gray
body, long wings : 1 gray body, vestigial wings : 1 black body,
long wings : 1 black body, vestigial wings, where both parental
types and non-parental types or recombinants are produced in
equal proportions).
26. • In test cross, non-parentals or recombinants (flies with gray
body , vestigial wings and black body , long wings) were not
produced because of complete linkage between the genes
for gray body colour and long wings and between black body
colour and vestigial wings in the F1 double heterozygote
male fly so that they remained together during inheritance
(GL/gl). Hence, there is lack of independent assortment
and crossing-over.
[Test cross to detect linkage: Test cross (Hybrid x homozygous recessive
parent) can be used to detect linkage. A deviation from the typical test
cross ratio (where both parental phenotypes and recombinant phenotypes
are formed in equal proportions) is an indication of linkage].
27. Complete Linkage in male Drosophila melanogaster
In this figure, the chromosomes are drawn as rod shaped .The results of the test cross between F1 male Drosophila
fly ( GgLl – can be represented as GL/gl in coupling or cis linkage phase, Gray long) with double recessive female
parent fly (ggll, Black vestigial) deviated from the expected dihybrid test cross ratio, 1:1:1:1(1 gray long [parentals] :
1 gray vestigial [recombinants] : 1 black long [recombinants] : 1 black vestigial [parentals] ). Instead , the progeny
include only parentals, Gray long and Black vestigial , produced in equal proportions. That is, 50 % Gray long : 50%
Black vestigial i.e. in a ratio, 1:1 and no recombinants are produced . This is due to complete linkage and hence
absence of crossing-over in F1 male Drosophila fly.
28. 2. Incomplete Linkage
• The genes that are distantly located in a chromosome are said
to be incompletely linked. This phenomenon is called
incomplete linkage (Partial linkage) .
• Incompletely linked genes in a chromosome have chances of
separation by a process called crossing-over.
• Most hybridizations involving incompletely linked genes
produce both parental types of progeny (in large numbers) and
non-parental types or recombinants (in small numbers - due to
certain degree of crossing-over between the linked genes) .
29. Example: Incomplete Linkage in female Drosophila melanogaster
• Calvin Bridges crossed a wild type female Drosophila fly with
gray body and long wings (Genotype GGLL) with a male
having black body and vestigial wings (Genotype ggll).
• The F1 flies were wild type with gray body and long wings
(Genotype GgLl, can be represented as GL/gl in coupling or
cis linkage phase).
• A test cross between F1 female fly and the double recessive
male parent fly produced progeny with more parental types
(gray long and black vestigial, 83 %) and a few non-parental
types or recombinants (gray vestigial and black long, 17 %)
and thus deviated from the typical dihybrid test cross
ratio,1: 1: 1: 1, where both parental types and recombinants
are produced in equal proportions(i.e., 1 gray long : 1 gray
vestigial : 1 black long : 1 black vestigial).
[ Wild type : The most common phenotype in a natural population].
30. • The deviation from the typical dihybrid test cross ratio,
1 : 1 : 1 : 1 and the occurrence of more parental types in the
test cross progeny shows that the genes are linked.
However, occurrence of a few non-parental types or
recombinants in the progeny shows that these genes are
incompletely or partially linked so that certain degree of
crossing-over occurred during meiosis which resulted in the
production of recombinant progeny.
31. Incomplete Linkage in female Drosophila melanogaster
The test cross between F1 female Drosophila fly ( GgLl , in coupling or cis phase, GL/gl , Gray long) with double recessive male fly (ggll,
Black vestigial) is shown. The presence of a few recombinants (Gray vestigial and Black long ) in the progeny indicates that , these
genes are not so tightly linked (Incompletely or partally linked ) and have undergone certain degree of crossing-over during Pachytene
stage of prophase of first meiotic division at the time of gametogenesis in F1 female fly. ( During Pachytene, each chromosome has 2
chromatids called sister chromatids . Crossing over occurs between non-sister chromatids).
32. • Thus, we have found that in Drosophila melanogaster, the
linkage between the factors or alleles for body colour (gray
vs. black) and wing size (long vs. vestigial ) were linked to
each other and the linkage is complete in the male fly and
incomplete in the female fly .
33. Significance of Linkage
• Linkage is the phenomenon where the genes located very closely in a
chromosome have a tendency to be inherited together as a single unit
during meiosis. This enables the preservation of parental gene
combinations which in turn help to maintain the constancy of a
species from generation to generation.
• Linkage play an important role in the preservation of beneficial
combinations of alleles as the tightly linked genes rarely undergo
crossing-over. Linkage between two or more genes in a chromosome
that control different desirable characters is beneficial for
simultaneous genetic improvement of two or more characters.
• On the other hand, in some chromosomes, a gene that control a
desirable character may be closely linked with another gene that is
responsible for an undesirable one. In such cases, along with the gene
for the desirable trait, the other gene for the undesirable trait may
also be inherited together, which may reduce the fitness of the
progeny (i.e., Linkage drag). Such a condition will be a hindrance for a
breeder to genetically improve those traits.
34. • Linkage analysis is an effective method to locate a gene of
interest in a chromosome . Linkage analysis has helped to
find out the location of several genes on human
chromosomes and to study the genetic basis of many
human diseases.
• Linkage restrict genetic variability in a population.
[Genetic variability is essential for hereditary improvement of traits].
35. Crossing-over
• Though all the genes in a chromosome are said to be linked,
there is a possibility for those genes which are distantly
located in a chromosome to get separated by crossing-over.
• Crossing-over occurs in all sexually reproducing organisms
during the first meiotic division.
36. • Crossing-over involves the reciprocal
exchange of equivalent portions
between non-sister chromatids of a pair
of homologous chromosomes. Morgan
and Cattell (1912) had called this process
of interchanging, "crossing-over".
• The crossing-over break the linkage
between genes that leads to shuffling
and genetic recombination. Therefore,
crossing-over results in new
combination of alleles in the gametes.
• That means, crossing-over alters the
pattern of genes in the chromosomes
and thus creates genetic variability
among population.
[Population : A group of organisms of one
species that interbreed and live in the same
place at the same time].
Image :https://www.toppr.com
37. Cytological basis of Crossing-over
(Physical mechanism of Crossing-over )
• Studies in Maize (Corn, Zea
mays) by Harriet
Baldwin Creighton and
Barbara Mc Clintock (1931)
and in Drosophila by Curt
Stern (1931) had proved that
crossing-over involves physical
exchange (physical swapping)
of segments between non-
sister chromatids of paired
homologous chromosomes.
Harriet Baldwin
Creighton
Barbara Mc Clintock &
Curt Jacob Stern
42. Joan E. Bailey-Wilson Ph.D. National Human Genome Research Institute
https://www.genome.gov/genetics-glossary/Crossing-Over
43.
44. Crossing-over results in the production of
recombinant chromosomes -
Leads to gene shuffling or genetic recombination
Image credit : Snustard & Simmons , 2012
45. • Sites of crossing-over (the point of
interchange of chromosomal segments
between non-sister chromatids) can be
cytologically recognized by a cross-
shaped configuration called chiasma
(Plural: Chiasmata - a term coined by
Frans Alfons Janssens,1909 - a Belgian
cytologist. (Janssens’ chiasmatype
hypothesis).
• In other words, chiasmata are the
cytological manifestation of crossing-
over.
Frans Alfons Janssens
46. • Chiasma may be either
terminal (located at the end)
or interstitial (located in the
middle part).
• Chiasmata can be seen during
the Diplotene (Diplonema)
stage of Prophase of the first
meiotic division. During
Diplotene stage, chromosomes
repel each other slightly,
maintaining close contact only
at the centromere and at each
chiasma. This partial
separation makes it possible to
count the chiasmata accurately.
• During Diplotene, the
interstitial chiasma begin to be
displaced along the length of
the chromosomes by a process
called terminalization.
uploaded by Liangran Zhang
https://biology4isc.weebly.com Diplotene
47. Types of Crossing-over
i) Single crossing-over
• In this type, a single
chiasma is formed at one
point between non-sister
chromatids of a pair of
homologous chromosomes.
That means, only one
crossing-over occurs.
• This results in the formation
of single crossover gametes.
Purves et al., Life: The Science of Biology,
4th Edition, by Sinauer Associates (www.sinauer.com)
and WH Freeman (www.whfreeman.com)
48. ii) Double crossing-over
• In this case, two chiasmata are
formed simultaneously between any
two points in the same pair of
homologous chromosomes. That
means, two crossing-over occur
simultaneously in a pair of
homologous chromosomes. This
process had been named by
Sturtevant as ‘double crossing-over’.
Double crossing-over occurs at a
frequency which is equal to (or less
than) the product of two single
crossing-over frequencies.
• Both the chiasmata may be formed
between the same chromatids or
between different chromatids. Thus
two (two-strand double crossing-
over), three (three-strand double
crossing-over) or four (four- strand
double crossing-over) chromatids of
the tetrad may be involved in the
process of double crossing-over.
Two-strand double crossing-over
(Here, only two strands of the tetrad are involved
in the formation of a double crossing-over.)
http://bio3400.nicerweb.net
50. iii) Multiple
Crossing-over
• Crossing-over
occurs at three
(triple crossing-
over), four
(Quadruple
crossing-over)
or more points
in the tetrad.
• Frequency of
multiple
crossing-over is
extremely low.
Snustard & Simmons , 2012
51. Factors affecting the frequency of crossing-over
• Frequency of crossing-over (i.e., the number of crossing-over at
meiosis I ) and recombination varies in chromosomes and also
among species.
• In most of the species, the number of crossing-overs is much
lower than the number of DNA double-strand breaks. This is
because, all DNA double-strand breaks may not be repaired as
crossovers. Some DNA double-strand breaks are repaired as
non-crossovers. For example, in the plant Arabidopsis thaliana,
out of 150 to 300 DNA double-strand breaks, only around 10 are
repaired as crossovers. Similarly in Maize, out of 500 DNA
double-strand breaks, only about 20 are repaired as crossovers.
In humans, approximately 10% of DNA double-strand breaks
are repaired as crossovers. The decision, which DNA double-
strand breaks will be repaired as crossovers and which ones
will be repaired as non-crossovers is called crossover-
designation.
52. • The number of crossing-over is strictly regulated to form an optimum
number of physical links (i.e., chiasmata) between homologous
chromosomes to ensure their accurate segregation during anaphase
of meiosis I.
• Even if there is fluctuation in DNA double-strand break formation, the
frequency of crossing-over is maintained more or less constant
(Crossover homeostasis).
• The formation of crossing-over is regulated in such a way that their
number is kept at a very low level. This may help to maintain
the DNA sequences in cells with very little change from generation to
generation. In most of the species, the average number of crossing-
over per chromosome rarely exceeds three per bivalent.
• However, several genetic, epigenetic (= relating to or arising from
non-genetic influences on gene expression e.g. DNA methylation) and
environmental factors can influence the frequency of crossing-over.
[DNA methylation : Addition of methyl (CH3) group from S-adenyl methionine to
the fifth carbon (C5 position) of a cytosine to form 5-methylcytosine in DNA.
Methylation can change the activity of a segment of DNA without changing the
sequence. DNA methylation play a role in epigenetic gene regulation.]
53. Let us consider some factors that influence the frequency of
crossing-over during meiosis.
Distance between linked genes
• Since genes are arranged linearly on chromosomes, the
strength of linkage depends upon the distance between the
genes involved. That means, genes that are located very
close to each other in a chromosome are strongly linked.
That is, there is high degree of linkage. Therefore, in closely
spaced gene loci, the probability of crossing-over will be
very less and they are less likely to recombine. So the
frequency of recombination might be very little.
55. • The percentage of crossing-over (recombination) or Cross over value can be
calculated as
Where, total number of progeny = Total No. of parental types + Total No.
of recombinants.
(As already stated, when there is random assortment (i.e. independent assortment)
of genes, the parental types and the recombinants will be formed in equal
proportions. That means in a 1:1 ratio).
On substitution, we get
1/(1+ 1) x 100 = 50%.
[i.e. 1(Total No. of Recombinants) / 1(Total No. of parental types) + 1 (Total
No. of recombinants ) x 100].
• Thus, the recombination frequency between two independently assorting
genes cannot exceed 50%. In other words, the maximum recombination
frequency between two independently assorting genes is 50%.
56. • Sturtevant(1914), had suggested that frequency of recombination could be
used to estimate the relative distance between two genes. 1% Recombination
frequency = 1 map unit = 1 cM. In other words, 1 cM indicates a 1%
recombination potential. That means, in 100 meioses, there will be one
recombination event .
• Thus, two genes with a low recombination frequency are likely to be closer
together while those with a high recombination frequency are likely to be
farther apart on a chromosome.
• Based on this, Sturtevant created the first genetic map ( Linkage map) of
genes on the X chromosome of Drosophila melanogaster.
[ cM – centiMorgan,(in honor of Thomas Hunt Morgan), the unit of map
distance.
Map distance : According to Sturtevant(1914), percentage of crossing-over is
used as an index of the distance between any two pairs of genes. That is, the
unit of distance is taken as a portion of the chromosome of such length that, on
an average, crossing-over occurs in it in 1% of the germ cells(reproductive cells).
i.e., one map unit is equal to 1% recombinant phenotypes. In other words, a
spacing of 1 cM indicates a 1 % chance that two genes will be separated by
crossing-over ].
57. Crossover Assurance, Interference and Coincidence
Crossover Assurance
• Irrespective of size, at least one crossing-over (obligate crossing-over)
occurs per bivalent, generating the ‘obligate chiasma’ that ensures
proper disjunction of homologous chromosomes during anaphase I
of Meiosis I. This phenomenon is called ‘Crossover Assurance’ .
Failure to maintain at least one crossing-over per homologous pair of
chromosomes increases the probability of non-disjunction which in
turn may result in the formation of aneuploid gametes.
[Disjunction : Normal separation of chromosomes toward opposite poles
during anaphase.
Non-disjunction : The failure of one or more pairs of homologous
chromosomes or sister chromatids to separate normally during nuclear
division, usually resulting in an abnormal distribution of chromosomes in the
daughter nuclei.
Aneuploidy : The loss or gain of one or a few chromosomes].
58. Interference (Crossover Interference or Genetic
Interference or Chiasma Interference)
• The occurrence of one crossover in a given chromosome pair
tends to prevent the occurrence of another one in that pair. This
phenomenon is called interference. In other words, crossing-over
in one region interferes with the crossing-over in the nearby
regions. That is, crossing-over at one point in a chromosome
reduces the probability of another crossing-over at a nearby
position in the same chromosome. This is because, the chiasma
formed during one crossing-over affects the ability to form
another chiasma in the nearby region.
• Thus crossover interference is a measure of the independence of
crossing-overs from each other and it is a patterning phenomenon
that ensures that crossing-overs are widely spaced along a
chromosome as well as a more or less even distribution of
chiasmata along chromosomes. That is, crossover interference is
non-random placement of crossovers along chromosomes.
• The net result of crossover interference is fewer double-
crossovers than would be expected according to map distances.
59. • One way in which genome-wide crossover reduction could be
achieved without forming univalents is by an increase in the
strength of crossover interference. Increasing interference could
reduce crossover numbers without affecting the formation of at
least the single ‘obligate’ crossover (Jones and Franklin, 2006). The
strength of interference varies in different regions of the
chromosome. Interference decreases with distances up to 46 map
units (centiMorgans) from the initial crossing-over .
• Crossover interference has been observed in most organisms.
• The level of crossover interference is influenced by sex, age,
length of chromosomes, etc. In mice and cattle, interference is
stronger in females than in males (Szatkiewicz et al., 2013; Wang
et.al., 2016) where as in humans, interference is stronger in males
than in females (Campbell et al., 2015).
• The exact mechanism by which interference is exerted is not fully
understood. It may be due to multiple levels of recombination
regulation.
60. Coincidence
• The simultaneous occurrence of two single crossing-over events in
the same pair of homologous chromosomes to form a double
crossing-over is called coincidence .
• Coefficient of coincidence (CoC ) = Observed frequency of double
crossovers / Expected frequency of double crossovers in target
intervals .
[ In other words, Number of double recombinants in the progeny /
Number of double recombinants expected in the progeny ;
Expected double crossovers can be calculated as
Expected double crossovers = Recombination frequency in region 1
X Recombination frequency in region 2. i.e., the probability of a
double crossover is the product of their separate probabilities].
• Coefficient of Coincidence (CoC) + Interference (I) = 1
Therefore, interference , I can be calculated as
I = 1 – CoC .
61. • When interference is complete, i.e., I = 1, CoC = 0. i.e., there
is no coincidence. That means, there will be no double
crossing-over. That is , a crossing-over in one region will
interfere the occurrence of another crossing-over in a
nearby region.
• Sturtevant had found that interference decreases further
away from the locus of the first crossing-over. Thus, double
crossing-over is much less likely to occur in short distances
than in longer ones. Therefore, strength of interference is a
function of map distance. Interference is stronger over short
distances (i.e., stronger over map distances less than 20cM)
and decreases if the gene loci are at a greater distance.
Thus, the intensity of interference is inversely proportional
to the distance between gene loci.
62. • At a certain map distance, interference disappears (i.e.,
interference, I = 0. That means, there is no interference. So,
the coincidence, CoC = 1. That is, coincidence is complete.
Hence double crossing-over will occur at the expected
frequency. That means, a crossing-over at one region occurs
independently of a crossing-over at a nearby region. Thus,
coincidence is an inverse measure of interference.
• If the value of coefficient of coincidence is between 0 and
1, there is partial interference and there will be a few
crossing-overs .
63. Position of gene
• The correlation between distance between genes and the probability of
crossing-over may not be applicable to all genes in a chromosome. This is
because, formation of chiasma does not occur at random throughout the
length of a chromosome.
• Crossing-over occur in preferential regions of the genome. That means,
certain regions of some chromosomes have significantly higher rates of
crossing-over or recombination called crossing-over hotspots or
recombination hotspots, which serve as localized stimulators of general
recombination and other regions which have unusually low recombination
called coldspots, which depress the level of recombination. Recombination
hotspots are interspersed with recombination coldspots.
[Crossing-over hotspots: DNA fragments of a few kilobases in length with a higher rate of
recombination than the surrounding DNA - Hotspots occur in very small regions, approximately
25% of the genome, where 80% of the crossing-over occur. For instance, in many primates, mice
etc., 80 % of recombination occur in 10-20 % of the genome. In humans, recombination hotspots
are approximately 10% of the genome where 40-60 % recombination occur. In many plant
species also, recombination events are not evenly distributed along the length of the
chromosomes. For instance, in wheat (Triticum aestivum L.) chromosome 3B, it has been
estimated that 90% of the crossing-over occur in distal sub-telomeric regions. In Arabidopsis,
crossing-over hotspots are closely associated with DNA double-strand break hotspots (Choi et. al.,
2018) ].
64. • Crossing-over is generally suppressed near the centromere
(Centromere effect - first described in Drosophila ) and
telomeres (Telomere effect) of a chromosome (Coldspots).
• The mechanism of suppression of crossing-over at centromere
is not fully understood. Presence of heterochromatin as well
as DNA methylation at centromere may result in the
suppression of crossing-over. If crossing-over occurs at
centromere region, it may cause aneuploidy during meiosis
in females. In regions adjacent to centromeres, double-strand
breaks may occur but they do not resolve into crossing-overs.
• The presence of large blocks of heterochromatin at telomere
may affect the formation of chiasma and thus suppress
crossing-over.
65. • Mostly crossing-over occur at chromosomal arm regions. In
most plant species, crossing-over mainly occur in distal
euchromatic regions than in central regions.
• Linkage analyses in Maize (Zea mays, Gore et. al., 2009) and
Wheat (Triticum aestivum, Saintenac et. al., 2009) revealed
that the frequency of crossing-over is higher in sub-
telomeric regions and lower in interstitial regions.
Size of chromosomes
• The length of a bivalent influences its ability to form
chiasmata. So, the frequency of crossing-over is higher in a
long bivalent than in a short one. Short bivalents exhibit
strong interference.
66. Chromatin Structure
• Frequency of crossing-over can be influenced by chromatin
structure. In Maize, frequency of crossing-over is more in the distal
gene-rich euchromatic regions.
• Crossing-over can be suppressed by methylation of DNA and
histones ; the amount and distribution of highly repetitive
heterochromatin etc. The occurrence of crossing-over is rare or
absent in heterochromatic regions. Genomic regions with
repetitive sequences have low frequency of crossing-over.
• Recombination frequency is positively correlated with G-C content
in certain organisms like Drosophila melanogaster, Saccharomyces
cerevisiae ( brewer's yeast or baker's yeast), honey bee etc. There
are also reports that regions with low G-C content have high
recombination (Lynn et.al., 2004). But, in Arabidopsis thaliana,
there is no correlation between G-C content and the frequency of
crossing-over.
[ G-for Guanine and C-for Cytosine, the nitrogen bases in DNA]
67. • Recombination frequency is also influenced by transposable
elements. Generally, crossing-over is suppressed in highly
repetitive genomic regions that are made up of transposable
elements (Underwood and Choi, 2019).
• Crossing-over hotspots identified from Maize have low DNA
methylation and transposons. High transposable element
densities have been reported in low recombination regions of
Drosophila species. On the other hand, Theobroma cacao and rice
populations show largely divergent hotspot locations influenced
by retrotransposon abundance and genetic divergence (Marand
et. al., 2019).
[ Transposable elements (Transposons or Jumping genes) : They can move
from one location on the genome to another - first discovered by Barbara
Mc Clintock in Maize .
Retrotransposon : They transpose through RNA intermediates. First, the
transposable DNA is copied into RNA and then into DNA by reverse
transcription by an enzyme reverse transcriptase and inserted into the
target site]
68. • Generally, there is a positive correlation between gene density and
frequency of crossing-over.
• Formation of crossing-over is affected by sequence homology. For
instance, in the case of inversion heterozygotes and translocation
heterozygotes, where one chromosome has the normal sequence and
the other chromosome with altered sequence, the pairing of
homologous chromosomes will be disrupted in the regions of
chromosomes where such changes have occurred , which will in turn
reduce the probability of crossing-over. That is, reduction in sequence
homology between chromosomes will lessen the probability of crossing-
over and thereby recombination.
[Gene density : The number of genes per million base pairs, called a
megabase, Mb ; For instance, the gene density of the human genome is
roughly 12–15 genes/Mb.]
Inversion : A type of structural change in a chromosome in which two
breaks occur and the broken segment is reinserted after rotating 180° so
that the gene order is reversed.
Translocation: A type of chromosomal aberration which involves the
transfer of a segment of a chromosome to a different part of the same
chromosome or to a different chromosome, which change the
arrangement of the genes ].
69. Autopolyploidy
• Autopolyploids generally exhibit reduced crossing-over rates
when compared with their diploids (Shaver, 1962; Watanabe,
1983; Gillies et al., 1987; Yant et al., 2013).
Sex of the individual
• In most of the organisms, crossing-over occur in both males and
females. But, there are exceptions. Crossing-over is absent in male
Drosophila melanogaster (Morgan, 1914 ; but Drosophila
ananassae males undergo a few crossing-over )and female silk
moth Bombyx mori (Tanaka, 1914) ( Achiasmy – Complete
suppression of recombination in one sex).
• Sex-specific differences in the frequency of crossing-over
(Heterochiasmy) and its distribution along chromosomes have
been reported in many species.
[Autopolyploidy : The phenomenon where all the genomes in a polyploid
organism are identical
Polyploid : An organism with more than two sets of chromosomes].
70. • Frequency of crossing-over may be
a) Higher in female meiosis – E.g. Eutherian mammals - For
instance, crossing-over frequency in oocytes of human females is
generally higher than in spermatocytes of human males,
which correlates with differences in synaptonemal complex length -
The synaptonemal complex is considerably longer in oocytes in
comparison to spermatocytes. (Tease and Hultén, 2004 ; Petkov
et.al., 2007).
b) Higher in male meiosis - In some metatherian mammals, Sheep,
Arabidopsis thaliana etc.
In the plant Arabidopsis thaliana, crossing-over rates in distal
regions of chromosome 4 are very high in male meiosis but very
low in female meiosis ( Drouaud et.al., 2007).
c) Almost same - There is no significant sex-specific differences in
the rate of crossing-over. e.g. Tomato, Barley, Rape seed or Canola
etc.
[Eutherian mammals : Placental mammals that give birth to well-developed
young ones.
Metatherian mammals : They give birth to partially developed young ones e.g.
Marsupiales].
71. Maternal Age
• The frequency of crossing-over and recombination is found to vary
among different age groups. The frequency of crossing-over may
either increase or decrease as the maternal age advances.
• A reduction in frequency of crossing-over with regard to advancing
maternal age was reported in oocytes of mice, Drosophila etc. A
decrease in the frequency of chiasmata, a change in their location on
the chromosome and an increase in the frequency of univalents have
been found in mice oocytes as the age advanced. (Henderson and
Edwards, 1968). In female Drosophila flies also, the frequency of
crossing-over decreased as the age advanced (Bridges,1927;
Whittinghill and Hinton, 1950) . They had reported that the highest
rate of crossing-over takes place in the eggs laid during the first 4 to 5
days and gradually declines till the 12th-16th day. Bridges found a
20% decline in recombination rate as the age advanced in female
Drosophila flies. Such changes in a chromosome may be due to local
rearrangements. However, an increase in crossing-over in older age
(> 16 days) has also been reported.
72. • In humans, both increase and
decrease in recombination
rates with advancing
maternal age have been
reported. In older women,
reduction in crossing-over
and genetic recombination
may cause non-disjunction
which in turn may result in
aneuploidy and may cause
many problems like
infertility, miscarriages, birth
defects(e.g. Down syndrome)
etc.
Down syndrome : Trisomy 21 – The
individual has three copies of the
chromosome 21 instead of the
normal two copies.
https://www.savedownsyndrome.com/blog/theneweducation
algraphics-csyye
73. Temperature
• The frequency of crossing-over may decrease or increase
with regard to variations in temperature.
• The effects of temperature on the frequency of crossing-over
and recombination are species-specific.
• In Rice plants, crossing-over and recombination increases
with a rise in temperature.
• In Wheat, high temperature within the fertility threshold
(i.e., between 10°C and 26°C) has a positive impact on the
frequency of crossing-over and thereby meiotic
recombination (Coulton et al., 2020).
74. • In Arabidopsis thaliana, both low (8°C) and high (28°C)
temperatures (Cold and heat stresses), increase the frequency
of crossing-over. An increase in temperature within the
fertility-tolerable range (28°C) promotes crossing-over. For
instance, frequency of crossing-over increased when
Arabidopsis plant grown at a temperature of 20°C was
shifted to 28°C. The frequency of crossing-over was
approximately 10% higher at the extreme of the
temperature range, 8–28 °C.
• An increase in chromosome axis length may account for the
rise in frequency of crossing-over at low temperature in
Arabidopsis thaliana (Lloyd et. al., 2018).
• A higher temperature (32°C - 38°C) disrupts central element
of synaptonemal complex and causes asynapsis and thus
affects bivalent and chiasma formation which in turn result
in the suppression of crossing-over (De Storme and Geelen,
2020).
[Asynapsis : Failure of pairing of homologous chromosomes]
75. • In Barley (Hordeum vulgare), changes in temperature from
22°C to 30°C caused reduction in frequency of crossing-
over. The distribution of crossing-over was also altered, and
there were significantly more crossing-over in the
interstitial regions at higher temperature (Higgins et. al.,
2012).
• However, Phillips et. al., (2015) have reported that in
Barley, heat causes an increase in the rate of crossing-over
and with a redistribution of crossovers from distal toward
more proximally located chromosome regions.
76. • In female Drosophila flies, the frequency of crossing-over is
less at temperatures between 22°C and 25°C. But, more
crossing-over is observed if the temperature is either
lowered or raised (Plough, 1917; Stern, 1926; Graubard,
1934 and Smith, 1936).
• Crossing-over was induced in male Drosophila flies at a
temperature of 35°C, applied during the larval period
Whittinghill (1937) .
• In Drosophila, heat shock results in crossovers on
chromosome 4.
• Position of chiasma (e.g. centromere proximal versus distal)
can also change with variations in temperature (Abel, 1964;
McNelly-Ingle et al., 2009).
77. Starvation or Nutritional deficiency
• Nutritional stress like starvation or nutritional deficiency increases
recombination in Drosophila melanogaster. Extreme changes in
larval nutrition affected the recombination frequency in the third
chromosome of Drosophila melanogaster (Neel, 1941).
• The ionic status of the cells of an organism also influences the
frequency of recombination. For instance, the presence of metallic
ions such as Calcium and Magnesium ions in the food reduced the
frequency of crossing-over and recombination in Drosophila. But,
the removal of such chemicals from the diet increased the rate of
crossing-over.
• High level of Sodium ions also caused reduction in crossing-over
(Griffing and Langridge 1963).
• Phosphate treatment increased chiasma number in both diploids
and tetraploids of the grass Festuca pratensis (Deniz and Tufan,
1998). High levels of phosphate increased chiasma frequency in
two strains of diploid rye (Secale cereale, Bennett and Rees ,1970).
78. Chemicals
• Treatment with mutagenic chemicals like alkylating agents was found to
increase the frequency of crossing-over in female Drosophila fly. Ethyl
methane sulphonate is known to induce somatic crossing-over.
• Exposure to Ethylene diamine tetraacetic acid (EDTA) increase the
recombination rate in female Drosophila (Levine, 1955).
• Colchicine prevents crossing-over by preventing synapsis (pairing of
homologous chromosomes).
• High dose of Selenium reduces the frequency of crossing-over.
• Antibiotics such as Mitomycin-C and Actinomycin-D increase the
frequency of crossing-over.
• The anticancer drug (the chemotherapeutic agent) Cisplatin (cis-
platinum(II)diamine dichloride) is highly recombinogenic in assays with
some model organisms such as Candida albicans, Saccharomyces
cerevisiae and somatic cells of Drosophila melanogaster. Cisplatin and
UV exposure cause different forms of DNA damage that can be
processed by the homologous recombination pathway and
form crossing-over during meiosis.
79. Plasma genes (Cytoplasmic Genes)
• In some species, plasma genes may cause reduction in
crossing-over. For example, Tifton male sterile cytoplasm
(Tift 23 A1 cytoplasm) in Pearl millet (Bajra - Pennisetum
glaucum).
[Tift 23 A : Cytoplasmic-genic male sterile (CMS) line of Pearl millet with
short stature, profuse tillering, uniform flowering and good combining
ability, evolved at Tifton, Georgia. In Pearl millet , the first reported CMS
system, A1 was based on the Tift 23A1 cytoplasm (Burton, 1965, Burton
and Athwal 1967) ; used in commercial hybrid seed production in Pearl
millet.]
80. Radiations
• Investigations on effect of radiation on frequency of crossing-over gave
conflicting results. The different responses of organisms to radiation may be
due to various factors such as biological ( i.e., species-specific differences
and variation in the developmental stage during which irradiation was done)
and physical ( i.e., nature of the radiation employed and the temperature
when irradiation was done).
• Generally, there was a decrease in the frequency of crossing-over after
irradiation.
• Plants irradiated just before the start of meiosis had much lower chiasma
frequency .
• Generally, crossing-over is absent in male Drosophila flies. But, extremely
low frequency of crossing-over had been reported in male Drosophila flies
(Muller, 1916; Bridges and Morgan, 1919 ; Sturtevant, 1929 ; Patterson and
Suche, 1933). It has been found that crossing-over can be induced in male
Drosophila flies by X-ray irradiation of immature germ cells. Rifenburgh
(1935) had reported that irradiation of young larvae by ultra-violet radiation
induced crossing-over between the black and vestigial loci in Drosophila
male fly.
[Irradiation : Exposure of a biological material to any one of the radiations ]
81. • X-ray and Gamma ray irradiation can increase the frequency
of crossing-over in female Drosophila fly.
• Jain and Basak (1965) had reported that radiation
treatments induced cryptic structural changes in some of the
chromosomes of Delphinium which restricted pairing which
in turn reduced chiasma frequency.
82. Significance of Crossing-over
1. Creation of Genetic variability
• Crossing-over and genetic recombination in the first meiotic
division during gametogenesis is an essential feature of sexual
reproduction that increase genetic variability among the
progeny, which is essential for effective selection (both
natural and artificial). Genetic variability is a prerequisite for
the evolutionary process.
• However, the low number of crossing-overs often limits the
genetic variation that can be utilized in breeding
programs(Plant Breeding and Animal husbandry).
[Plant Breeding : Applied branch of Botany that deals with the genetic
improvement of crops for the service of man.
Animal husbandry : The science of breeding and caring of domesticated animals].
83. 2. Increase /decrease of fitness and adaptability of the
progeny
Crossing-over and recombination may bring together the beneficial
or desirable alleles from both the parents which equip the progeny
with either more vigour, reproductive potential, survival ability or
adaptability. Such beneficial combinations of alleles may spread
through a population in several generations and eventually, may
become the distinctive features of the species. On the other hand,
crossing-over and recombination may break down the association
between certain beneficial alleles, which may reduce the vigour or
fitness and adaptability of the progeny.
• In some other cases, crossing-over and recombination may break
the association between one beneficial and another deleterious
alleles allowing selection to take advantage of the beneficial one.
• In crossing-over poor regions, desirable combinations of alleles are
preserved but it will be very difficult to get rid of the undesirable
ones.
84. 3. Regular or irregular segregation of paired homologous
chromosomes(Bivalents) at Anaphase I during Meiosis I
• Crossing-over is required for the normal segregation of pairs of
homologous chromosomes during anaphase I of meiosis I.
Failure to maintain at least one crossing-over per homologous
pair of chromosomes(i.e., obligate crossing-over) increases the
probability of non-disjunction. This is because, the chiasma
formed during crossing-over provide the physical attachment
between the paired homologous chromosomes which is
necessary for the proper alignment of the bivalents on the
metaphase I plate and their normal segregation at anaphase
I. Absence of chiasmata or inappropriately located chiasmata
may result in non-disjunction which in turn may cause
aneuploid variations in chromosome number and
consequently form aneuploid gametes.
85. 4. Preparation of Linkage maps
(Chromosome maps, Genetic maps or Crossover maps)
• Under standard environmental conditions, the recombination
frequency of a pair of linked genes is constant and characteristic for
that pair of genes. So, the frequency of crossing-over or
recombination or crossover value between gene pairs on the same
chromosomes can give an estimate of the relative distance between
them. As we have already found, 1% Recombination frequency = 1
centiMorgan (cM) = 1 map unit.
• Thus, the frequency of crossing-over or recombination between
different genes on a chromosome can be find out and can be used to
estimate their relative distances and order. Based on this data, we
can construct a linkage map. It is a linear representation of the gene
order and relative distance on a chromosome. That is, chromosome
maps are diagrammatic representation of chromosomes in the form
of a straight line showing genes as points separated by distance
proportional to the crossover value. In chromosome maps, one
member of a homologous pair of chromosome is represented as a
straight line proportional to its length with the position of genes
marked on them.
86. Example : Bridges and Olbrycht’s map of seven X-linked genes in Drosophila .
Map distances in centimorgans (cM)
Snustard & Simmons , 2012
87. Mitotic Crossing-over
• Crossing-over that occurs in somatic cells (i.e., body cells)
during mitosis (i.e., somatic crossing-over). It is a rare event
that occurs with a frequency of 10-4 to 10-5 per cell division
(Gunther, 1984).
• Mitotic crossing-over was first reported by Curt Stern (1936) on
X chromosome of a female Drosophila melanogaster fly with
the heterozygous genotype +sn/y+ (Repulsion or Trans linkage
phase) for body colour (wild type, gray ‘+’ vs. yellow, ‘y’) and
bristles ( normal ‘+’ vs. singed ‘sn’ – gnarled or short twisted
bristles). That is, one X chromosome carry the recessive allele
‘y’ for yellow body and the other X chromosome carry the
recessive allele ‘sn’ for singed bristles.
[Female Drosophila fly has two X chromosomes. i.e., XX].
88. • Stern had observed that most of the female Drosophila flies were
gray bodied and with normal bristles, as gray body colour is
dominant over yellow and normal bristles is dominant over
singed. However, Stern had also observed that some female flies
had single yellow spots, singed spots and twin yellow-singed
spots(i.e.,twin sectors, yellow adjacent to singed) on gray body.
This is because of mitotic crossing-over and segregation that
occured in a cell with heterozygous genotype, +sn/y+, which
results in cells homozygous for y and for sn. Thus mitotic
recombination resulted in the expression of recessive genes in
small areas in wild type gray bodied female fly.
[Twin spots : Paired alterations visible in adjacent areas – mosaic patches or
spots.
Organisms that are composed of cells of more than one genotype are referred
to as genetic mosaics].
90. • Jones(1937) had reported paired alterations visible in
adjacent areas(twin spots) and unpaired spots in the
aleurone layer of the endosperm of maize.
• According to Jones(1937), the twin spots in the triploid
aleurone layer of the maize endosperm is the result of a shift
of known color and texture genes C, C', Pr, P, Wx and Su due
to somatic crossing-over.
• Later, mitotic crossing-over had been reported in a wide
variety of organisms such as Aspergillus nidulans,
Saccharomyces cerevisiae, Nicotiana tabacum, Antirrhinum
majus, humans etc.