Linkage and Crossing-over.pptx

LINKAGE AND CROSSING-OVER
Dr Saji Mariam George
Associate Professor (Retired)
Assumption College Autonomous
Changanacherry

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

• British geneticists, Bateson,
Saunders and Punnett (1906)
had carried out hybridization
experiments in two varieties of
Sweet pea (Lathyrus odoratus).
• In this experiment, they had
focused the inheritance of two
pairs of contrasting characters
such as flower colour (purple
vs. red) and shape of pollen
grains (long vs. round), [a
dihybrid cross]. One Sweet
pea plant was with purple
flowers and long pollen grains
and the other with red flowers
and round pollen grains.
Sweet pea
(Lathyrus odoratus)
Edith Rebecca Saunders, William Bateson &
Reginald Crundall Punnett
Creative Commons Attribution 4.0 International ,https://www.researchgate.net , © 2014 Cornell University,
https://pollen.tstebler.ch
Pollen grains of
Sweet pea

• 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]

• 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.

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.

• 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)

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.

• 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 .

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]

• 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.

• 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.

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.

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.

• 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)

• 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).

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

• 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.

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.

 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.

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

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

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 ]

• 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).

• 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).

• 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].

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.

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) .

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].

• 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.

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).

• 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 .

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.

• 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].

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.

• 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

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

© 2014 Nature Education Adapted from Pierce, Benjamin. Genetics: A Conceptual
Approach, 2nd ed.
https://www.nature.com

The major cytological events that
lead to the occurrence of crossing-
over are the following.
• Pairing of homologous
chromosomes known as synapsis
occurs during Zygotene
(Zygonema) stage of Prophase of
the first meiotic division.
• Paired homologous chromosomes
form a bivalent. The two
homologous chromosomes do not
fuse during pairing but remain
separated by a space which is
filled with a proteinaceous
substance called synaptonemal
complex.
© 2014 Nature Education Adapted from Pierce,
Benjamin. Genetics: A Conceptual Approach, 2nd
ed.
https://biology4isc.weebly.com Zygotene

• During the next stage Pachytene (Pachynema), the
chromosomes become shorter and thicker. Each
chromosome split length wise (divide) to form two
chromatids (sister chromatids), held together by the
undivided centromere. Thus , the bivalent consists
of four chromatids (four strand stage or tetrad).
Crossing-over occurs at this stage.
• Crossing-over takes place by symmetrical breakage,
reciprocal exchange and reattachment of the pieces
between any two non-sister chromatids of the
tetrad.
[Double-strand breaks: Programmed double-strand
breaks by an endonuclease, Spo11(DNA
topoisomerase VI subunit A) at specific chromosomal
positions into segments – Formation of double-
strand breaks does not occur uniformly across the
genome. There are certain regions which are
favorable for double-strand break formation known
as double-strand break hotspots such as intergenic
and/or promoter regions where nucleosomes are
more widely dispersed . The union of segments of
chromosomes is catalysed by an enzyme , DNA
ligase.
Hotspots : Location of some intense activity]
© 2014 Nature Education Adapted
from Pierce, Benjamin. Genetics: A
Conceptual Approach, 2nd ed.
https://biology4isc.weebly.com
Pachytene

• Breakage and union between any
two of non-sister chromatids result
in two sets of chromatids , one set
with unaltered parental gene
combinations called the non-cross
over chromatids or non-
recombinant chromatids or non-
crossovers and the other two with
altered gene combination called
crossover chromatids or non-
parental or recombinant
chromatids or crossovers where
shuffling of genes occurred.
• As a result of crossing-over, non-
parental combinations of genes
appear in the gametes. Such
gametes are called crossover
gametes or recombinant gametes.
© 2014 Nature Education Adapted
from Pierce, Benjamin. Genetics: A
Conceptual Approach, 2nd ed.

Joan E. Bailey-Wilson Ph.D. National Human Genome Research Institute
https://www.genome.gov/genetics-glossary/Crossing-Over

Crossing-over results in the production of
recombinant chromosomes -
Leads to gene shuffling or genetic recombination
Image credit : Snustard & Simmons , 2012

• 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

• 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

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)

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

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

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.

• 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.]

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.

Copyright © 1997. Phillip McClean
• If the distance between two linked genes is more,
the strength of linkage decreases and there is
greater chance of occurrence of crossing-over. In
other words, the probability of crossing-over
between two genes is directly proportional to the
distance between them. i.e., greater the distance
between the two genes, the greater will be the
probability of formation of crossing-over.
• If the genes are located very far away, there is the
possibility of double cross-over, triple cross-over
etc. and the frequency of recombination may
also increase.
• Random assortment of genes will occur if the
two genes are far apart on the same
chromosome(or on separate chromosomes) and
may produce 50% recombination (i.e., The
progeny will contain both parental types and
recombinants in equal proportions. That means
in a 1:1 ratio). Thus, the strength of linkage is
inversely proportional to the distance between
two genes.

• 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%.

• 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 ].

 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].

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.

• 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.

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 .

• 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.

• 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 .

 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) ].

• 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.

• 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.

 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]

• 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]

• 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 ].

 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].

• 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].

 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.

• 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

 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).

• 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]

• 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.

• 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).

 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).

 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.

 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.]

 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 ]

• 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.

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].

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.

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.

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.

Example : Bridges and Olbrycht’s map of seven X-linked genes in Drosophila .
Map distances in centimorgans (cM)
Snustard & Simmons , 2012

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].

• 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].

Gray-bodied Drosophila
Yellow- bodied
Mutation - Singed bristle
, sn (short twisted bristles)
Image credit: https://www.carolina.com
Botaurus.
https://researchguides.library.vanderbilt.edu
https://www.biologie.uni-halle.de
http://rose-annemeissner.blogspot.com
Normal bristle

• 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.

Linkage and Crossing-over.pptx

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