Linkage and Crossing over (Sanjay Chetry).pptxsanjaychetry2
Linkage
1. Linkage ensures to keep the genes in a chromosome to inherit together
2. The strength of linkage between two genes is inversely proportional to the distance between them in the chromosome
3. The strength of linkage between two genes increases with the decrease in distance between them.
4. The strength of linkage decrease with increase in distance between the genes.
5. Linkage ensures the maintenance of parental trait in the offspring.
6. Linkage reduces the chance of creation of variability with sexual reproduction.
Crossing Over
1. Crossing over facilitates the separation of genes present chromosome and segregate into different gametes.
2. The chance of crossing over between two genes is directly proportional to the distance between them in the chromosome
3. The chance of crossing over between two genes decreases with the decrease in the distance between them.
4. The chance of crossing increases with increase in distance between the genes.
5. The crossing over causes alterations in the parental traits in the offspring.
6. Crossing over increases the chance of variability with sexual reproduction.
Linkage and Crossing over (Sanjay Chetry).pptxsanjaychetry2
Linkage
1. Linkage ensures to keep the genes in a chromosome to inherit together
2. The strength of linkage between two genes is inversely proportional to the distance between them in the chromosome
3. The strength of linkage between two genes increases with the decrease in distance between them.
4. The strength of linkage decrease with increase in distance between the genes.
5. Linkage ensures the maintenance of parental trait in the offspring.
6. Linkage reduces the chance of creation of variability with sexual reproduction.
Crossing Over
1. Crossing over facilitates the separation of genes present chromosome and segregate into different gametes.
2. The chance of crossing over between two genes is directly proportional to the distance between them in the chromosome
3. The chance of crossing over between two genes decreases with the decrease in the distance between them.
4. The chance of crossing increases with increase in distance between the genes.
5. The crossing over causes alterations in the parental traits in the offspring.
6. Crossing over increases the chance of variability with sexual reproduction.
Pleiotropy describes the genetic effect of a single gene on multiple phenotypic traits. The underlying mechanism is genes that code for a product that is either used by various cells or has a cascade-like signaling function that affects various targets.
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.
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.
Introduction :
Mendel and subsequent workers assumed that a character was governed by a single gene.
But it was later discovered that many characters in almost all the organisms are governed by two or more genes. Such gene affect the development of concerned characters in various ways.
The phenomenon of two or more gene affecting the expression of each other in various ways in the development of a single character of on organism is known as gene interaction.
Inheritance due to genes located in cytoplasm is called cytoplasmic inheritance.
Since genes governing traits showing cytoplasmic inheritance are located outside the nucleus and in the cytoplasm, they are referred to as plasmagenes.
Introduction, Types-somatic and germinal; Mechanism of meiotic crossing oversynapsis, duplication of chromosomes, breakage and union, terminalization;
Cytological basis of crossing over - Stern’s experiment in Drosophila; Creighton
and McClintock’s experiment in Maize; Crossing over in Drosophila, Construction
of genetic maps in Drosophila - two point and three-point crosses; Interference and
coincidence.
Pleiotropy describes the genetic effect of a single gene on multiple phenotypic traits. The underlying mechanism is genes that code for a product that is either used by various cells or has a cascade-like signaling function that affects various targets.
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.
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.
Introduction :
Mendel and subsequent workers assumed that a character was governed by a single gene.
But it was later discovered that many characters in almost all the organisms are governed by two or more genes. Such gene affect the development of concerned characters in various ways.
The phenomenon of two or more gene affecting the expression of each other in various ways in the development of a single character of on organism is known as gene interaction.
Inheritance due to genes located in cytoplasm is called cytoplasmic inheritance.
Since genes governing traits showing cytoplasmic inheritance are located outside the nucleus and in the cytoplasm, they are referred to as plasmagenes.
Introduction, Types-somatic and germinal; Mechanism of meiotic crossing oversynapsis, duplication of chromosomes, breakage and union, terminalization;
Cytological basis of crossing over - Stern’s experiment in Drosophila; Creighton
and McClintock’s experiment in Maize; Crossing over in Drosophila, Construction
of genetic maps in Drosophila - two point and three-point crosses; Interference and
coincidence.
Chromosomal crossover, is the exchange of genetic material during sexual reproduction between two homologous chromosomes' non-sister chromatids that results in recombinant chromosomes.
Contribution of crossing over and random assortment toAnna Purna
According to Darwin,Genetic diversity leads to evolution through natural selection. Meiosis contributes towards the genetic diversity through crossing over and random assortment. Random fusion of gametes also leads to genetic diversity.
Linkage and crossing over , discovery of linked genes,types of crossing over,significance and difference between linkage and crossing over, complete presentation with suitable examples and figures
Linkage
Genes far apart on the same assort independently are not linked
The position of the gene – locus
Occurs in the prophase of meiosis 1 where homologous chromosomes break at identical locations and rejoin with each other
Two genes are said to be under linkage, or linked, when they are located on the same chromosome.
Example: peas T=tall; t=short R=red; r=white
Crossing Over
Crossing over is a recombination of genes due to exchange of genetic material between two homologous chromosomes
It is the mutual exchange of segments of genetic material between non-sister chromatids of two homologous chromosomes, so as to produce re-combinations or new combinations of genes.
It occurs in the pachytene stage, at four strand stage with the help of enzymes (endonuclease, exo-nuclease, R-protein or recombinase;
Stern and Hotta,(1969, 1978).
There is breakage of chromatid segments, exchange of nonsister chromatid segments and later their fusion in new places.
Basics of Undergraduate/university fellows
Crossing over is exchange of strictly homologous segments of a genome between their
respective non-sister chromatids during cell division, which results in chromosomal
recombinations of linked genes in daughter cells.
The Art Pastor's Guide to Sabbath | Steve ThomasonSteve Thomason
What is the purpose of the Sabbath Law in the Torah. It is interesting to compare how the context of the law shifts from Exodus to Deuteronomy. Who gets to rest, and why?
This is a presentation by Dada Robert in a Your Skill Boost masterclass organised by the Excellence Foundation for South Sudan (EFSS) on Saturday, the 25th and Sunday, the 26th of May 2024.
He discussed the concept of quality improvement, emphasizing its applicability to various aspects of life, including personal, project, and program improvements. He defined quality as doing the right thing at the right time in the right way to achieve the best possible results and discussed the concept of the "gap" between what we know and what we do, and how this gap represents the areas we need to improve. He explained the scientific approach to quality improvement, which involves systematic performance analysis, testing and learning, and implementing change ideas. He also highlighted the importance of client focus and a team approach to quality improvement.
Model Attribute Check Company Auto PropertyCeline George
In Odoo, the multi-company feature allows you to manage multiple companies within a single Odoo database instance. Each company can have its own configurations while still sharing common resources such as products, customers, and suppliers.
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.
2. Genetics is the study of heredity and variations. Linkage and
Crossing over are the two main principles of inheritance and
variations.
The concept of linkage deals with the detailed study of gene
location on the chromosome and their inheritance that reflect
in the phenotype of an organism.
Linkage keeps the two or more linked genes together over
generations during inheritance.
3. Linkage can be complete or incomplete. The early
experiments to understand the phenomenon of linkage
were studied using Drosophila and sweet pea.
Crossing over is the phenomenon of the exchange of
genes through the recombination of sister chromatids. It
is of great importance due as it leads to variations in the
organisms.
4.
5.
6. Crossing over –
The exchange of chromosomal segments between
two non-sister chromatids of non homologous
chromosomes after crossing over.
F. Janssens was the first person to discover chiasma
formation and the related process of crossing over.
Morgan found the phenomenon of linkage and
recombination.
7.
8. Alleles-Different versions (sequences) of a gene.
Mutant-Newly created allele made by mutagenesis.
Genotype-The complete set of alleles for all genes carried by an
individual.
Wild type-Standard reference genotype. Most common allele for
a certain trait.
Phenotype-Observable trait specified by the genotype.
Point mutation-A change in a single base pair.
Silent mutation-A point mutation in a codon that does not change
the specified amino acid.
Genetics Terms
9. Crossing-over is a physical exchange between
chromatids in a pair of homologous chromosomes. It
results in a new association of genes in the same
chromosome.
The role of crossing-over is important for evolution to
take place. In fact, crossing-over and independent
assortment are mechanisms that produce new combinations
of genes.
Natural selection can then act to preserve those
combinations that produce organisms with maximum
fitness, that is, maximum probability of perpetuation of the
genotype
10. The Concept of Crossing-Over
Following are the important features of crossing-
over:
i) A gene is located on chromosomes at a
particular site called a locus (plural-loci).
ii) The loci of the genes on a chromosomes are
arranged in a linear sequence.
iii) In a heterozygote, the two alleles of a gene
occupy corresponding positions in the
homologous chromosomes, that is, allele A
occupies the same position in homolog 1 that
allele a occupies of a species is fixed or
constant.
11.
12. iv) Crossing-over involves the breakage and
rejoining of two chromatids (of homologous
chromosomes), resulting in reciprocal exchange of
equal and corresponding segment between them
v) Chromosomes with recombined or new
combinations of genes are formed by the
occurrence of crossing-over.
vi) Crossing-over occurs more or less at random
along the length of a chromosome pair. Thus, the
probability of its occurrence between two genes
increases with increasing physical separation of the
genes along the chromosome.
13. Crossing over refers to the exchange of genetic material
or chromosome segments between non-sister chromatids
in meiosis. This genetic process occurs between
homologous regions of matching chromosomes and the
interchange of homologous chromosomes.
Depending upon the number of chiasmata that appeared,
crossing-over can be categorised into three types:
Single cross-over
Double cross-over
Multiple cross-over
Significance of crossing-over are:
14. Crossing over produces a new combination of
genes
Crossing over plays an essential role in the process
of evolution
Crossing over frequency helps in the construction
of genetic maps
Crossing over provides the evidence for a linear
arrangement of linked genes in a chromosome
Crossing over provides an inexhaustible store of
gene variability in sexually reproducing organisms
15. Crossing over can put new alleles together in
combination on the same chromosome, causing them
to go into the same gamete.
When genes are far apart, crossing over happens
often enough that all types of gametes are produced
with 25% frequency.
16. Theories of Crossing Over
There are two theories to explain the relationship
between crossing over and chiasma formation:
1. Classical theory: It is also called a two-plane theory.
This theory was proposed by L.W. Sharp.
2. This theory stated that chiasma is the cause of
crossing over but not the result of crossing over. The
formation of chiasma occurs before the genetic
crossing over.
17. 3. Chiasma type theory: It is also called one plane
theory. It was proposed by Jannsen and later
developed by Belling and Darlington.
According to this theory, chiasma is the result of
crossing over, and crossing over precedes the
chiasma formation. This theory is widely accepted.
18. Mechanism of Crossing Over
The mechanism of crossing over involves the
following steps:
1. Synapsis: The pairing between the
homologous chromosomes (synapsis) takes place
during zygotene. This pair of homologous
chromosomes is called synapsis.
19. 2. Tetrad formation: The two chromatids of a chromosome
are referred to as dyads. A group of four homologous
chromatids (two dyads) of two synapsed homologous
chromosomes is known as a tetrad. The two chromatids of the
same chromosome are called sister chromatids. The two
chromatids, one of the one chromosome and the other of its
homologue, are termed non-sister chromatids.
A highly organized structure of filaments is formed between
the paired homologous chromosomes at the zygotene stage of
meiosis-I called synaptonemal complex. It helps in keeping the
homologous chromosome in a closely paired state.
20. 3. Exchange of Chromatid segments:
The two non-sister chromatids come in contact at certain points.
This is the region where the exchange of genes between the two
non-sister chromatids of a tetrad takes place. The places where
homologous chromosomes are held together and exchange bits
of chromatids are known as chiasma.
In synapsis, the non-sister chromatids of homologous
chromosomes break and recombine. This leads to the formation
of chiasmata. The exchange of fragments is stimulated by the
development of recombination nodules during the pachytene
stage. The unchanged part of the chromatid is called non-
crossover, and the changed parts are called recombinants.
21.
22. 4. Terminalization: The chromatids separate progressively
from the centromere towards the chiasma and get separated
from each other. It is called terminalization. Terminalization of
chiasma begins in the diplotene stage after crossing over, and
completion takes place in the diakinesis stage.
23. Significance of Crossing Over
The phenomenon of crossing over is of great significance that
can be discussed as follows:
1. It provides an inexhaustible store of genetic variability in
sexually reproducing organisms.
2. Since crossing over helps in the development of new
characteristics. Therefore it is of paramount importance in
plant breeding.
3. The new gene combination produced during crossing over
plays an important role in microevolution.
4. The frequency of crossing over is helpful in the mapping of
chromosomes.
5. Crossing over also justifies the linear arrangement of genes.
