1. Linkage refers to the tendency of genes located on the same chromosome to be inherited together from one generation to the next. Linked genes are housed in the same pair of chromosomes and are transmitted together more often than not. 2. The degree of linkage depends on the distance between genes - the closer the genes are on the chromosome, the stronger the linkage. Linkage can be classified as complete or incomplete depending on whether recombinant offspring are observed. 3. Linkage maps show the linear order and distances between linked genes on a chromosome based on recombination frequencies observed in test crosses. The number of linkage groups in an organism equals its haploid number of chromosomes.

1. LINKAGE-TENDENCY TO STAY
TOGETHER

2. PRESENTED BY
N. Sannigrahi, Associate
Professor,
Department of Botany
Nistarini College, Purulia (W.B)
India

3. Tendency to stay together is an inherent practice irrespective of the
organisms either forming a colony or to make a family as a draught
of bonding among the constituents members . This is not very
unnatural one and the nature gives us the principles to stay together
to enjoy a calm and peace in everyday’s hectic schedule. Turn eyes
towards the cell, a living machine. Thousand of characters are
regulated by hundreds of genes and these genes are housed in
nucleus in general and the chromosomes in particular. It is mostly
obvious that the chromosomes will carry many genes as the
number of chromosomes is very least in comparison to the number
of genes retained in organism. The diploid number of human
beings only 46 (23 pairs) but the 23 pairs of chromosomes harbors
near about 30000 genes altogether. Genes in the same chromosome
are tied to one another and being on the same chromosome would
move to the same pole during meiosis as a part of the formation of
gametes for the sexual reproduction. As a consequence, such genes
fail to assort independently and tends to be inherited together.

4. Genes located in non-homologous chromosomes undergo
independent assortment and produce Mendelian ratio. A significant
deviation from independent assortment was first reported by
Bateson & Punnet in 1905 for flower color and pollen shape in pea.
Instead of expected Mendelian test cross ratio of 1:1:1:1 , he
observed 7:1:1:7 .Bateson & Punnet proposed two new terms –
Coupling & repulsion phase . Later on, Morgan observed an
interesting genetic attributes as far as the inheritance of characters
from one generation to the subsequent generation-called Linkage.
Certain genes located on the same chromosome tend to remain
linked together while passing from one generation to another. This
tendency of certain genes to stick together in a way in which they
entered the cross has been termed as linkage and such genes
associated are called linked genes. The most fascination drama of
the biology later on drew the attention of the geneticists in this
regard.

5. CHARACTERISTICS OF LINKAGE
1.Linkage is an exception of Mendel’s independent assortment
rules,
2.Linked genes are housed in the same pair of chromosomes,
3.Lesser the distance between the linked genes, stronger the
linkage bonds, Genes lying far part enjoy least amount of linkage
strength and the chance of separation is higher during biological
incidents,
4.Linked genes transmitted together from one generation to the
nest generation mostly in an undisrupted fashion,
5.Separation of linked genes are a seldom events in biology.
Thus , linkage is an indication of the strong association of the
genes, the unit of characters from generation to generation

7. LINKAGE GROUPS
The number of the linkage groups in a species, as a rule, equal to
the number of the haploid number of the organisms. If a certain
gene ‘x’ is linked to two other genes ‘y’ & z’, then it is true that y
& z are linked. I plants or animals where several genes exist,
crosses are arranged to ascertain the existence of linkage of pairs
or of groups of several pairs of genes. The genes known to exist
in a species and are thus divided into linkage groups. Mostly the
haploid number along with the sex chromosomes generally build
up the number of minimum possible linkage groups of higher
organisms including humans.
In drosophila, number of linkage groups are 5,
Barley having 7,
Pea 7 linkage groups,
Man having 24 and in mouse , it is 13.

8. COUPLING & REPULSION PHASE
In the dihybrid test cross of the Mendelian traits, the expected
ratio is 1:1:1:1 but in some cases the chance of parental type is
much more than the expected ratio. This can be explained by the
coupling & repulsion phase in order to elucidate the cause and
the consequences of the phenomenon of linkage .
COUPLING PHASE
The tendency to inherit together of the genes with close
association is called coupling( As in the two bogies of the train is
connected with coupler) and the chance of separation of the far
apart is called as Repulsion as stated below. In maize, a dominant
gene C produces colored seeds while recessive c produces
colorless and another dominant gene Sh produces full seeds while
sh produces shrunken. A cross between CCShSh and ccshsh
produces all F1 as colored and full seeds but when the F1
ubndergoes test cross with the double recessive, ccshsh, it
produces a very different combinations like 48.2% colored full,

9. 48.3% colorless shrunken, 1.7% colored shrunken and 1.8%
colorless full and this ratio does not reflect with the expected ratio
of 1:1:1:1 .The parental type has more frequencies than the
recombinant types, colorless full and colored shrunken. In the
above examples, it appears as if two dominant gene3sC & Sh have
a strong affinity with each other so that the frequencies of the
colored full and colorless shrunken phenotypes are much more
expected than the recombinant types. This is referred as Coupling
phase and in this consequence, it is due to C & Sh in the same
chromosome and they are markedly enjoying a strong bond of
linkage upon each other.
REPULSION PHASE
In the another consequence, when plants with colored shrunken
seeds ( CCshsh) were crossed with colorless full seeds(ccShSh),
the F1 sees were colored full ( CCShSh) but when the F1 plants
were undergone test cross, 47.9% colored shrunken, 49.1%
colorless full,1.4% colored full and 1.5% colorless shrunken. Here
also parental type appears more frequent than recombinant types

10. The situation is referred as repulsion phase. Needless to say,
coupling and the repulsion phases are only two situations of the
same phenomenon of linkage. The coupling and the repulsion both
maintain the law of independent assortment. It is due to the
presence of the dominant one gene with the recessive allele of the
other. Obviously, Coupling and repulsion are two situations of the
same type of genetically behaviors.

12. COMPLETE & INCOMPLETE LINKAGE
Genes show linkage as because they are located on the
same chromosome. In the previous content, it has been
observed that the genes C and Sh are present on the
same chromosome while their recessive alleles , c & sh
are present on the different alleles on the same
homologous chromosomes. Now each chromosome
behaves as a unit during the cell division in general and
karyokinesis in particular. Therefore, genes C and Sh
would move to one pole while c& sh moves to the
another pole in opposite direction as per the rule of
nuclear division during anaphase. Therefore in F1, it
produces CcShsh and it the test cross progeny appears
two classes.

13. This is called complete linkage but when the recombinant types are
also recovered in the test cross progeny, it is called incomplete
linkage. Linkage is also classified as
Coupling and
Repulsion phase linkage.
In coupling phase, the dominant alleles of the linked genes are
present in the same chromosome but in contrast, in repulsion phase
of linkage, the dominant allele of one gene present with the
recessive allele of the other gene of the same chromosome.
The method of the representation of the linkage may be different
types. Mostly the linked genes written in Csh/cSh or CSh/csh but
very often they are denoted by CSh// csh , the two lines represent the
two chromosome in which the genes are located.

15. RELATIONSHIP BETWEEN LINKAGE & CROSSING
OVER
• Linkage and Crossing over are mostly advocated issues
discussed in biology and they enjoy the relationship as stated
below:
• 1. Linkage is an exception of Mendel’s principles of
independent assortment , the crossing over is also the
exceptions of Mendel’s independent assortment,
• 2. Linkage and Crossing over enjoy an inverse relationship.
The more distance between the two genes on a chromosome, it
decreases the strength of linkage but increases the chance of
the crossing over more. More the linkage, the least the chance
of crossing over.
• 3.Crossing over occurs between the two non-sister chromatids
of the homologous chromosome but in linkage it is not
involved.
• New combinations of the genes are the outcome of crossing
over but old combination is retained by linkage.

16. DETECTION OF LINKAGE
• Linkage between two dominant genes produces significant
variation both in the test cross ratio of Mendel’s dihybrid
outcome(1:1:1:1) and the result of F2 phenotypic ratio of
dihybrid result (9:3:3:1) and this data is very significant
enough for the assessment of the linkage of the concerned
groups of organisms for the consequence of the detection of
the linkage phenomenon. In case of linkage, the two parental
types are most frequent , they are in excess of the expected
25% for each parental type. In contrast, the two recombinant
types have markedly lower frequencies than 25% each. Thus,
it is typical of linkage that two phenotypic classes are
markedly greater than 25% of each, while the remaining two
phenomenon are markedly lower than 25% each. The
following example can explore the beauty of the
consequences.

17. DETECTION OF LINKAGE
• In the crosses between colored and full with colorless and
shrunken pea seeds, the F1 appears all colored full but the
testy cross result exhibits the following combinations-
• Colored full -----4032
• Colorless shrunken------4035
• Colored shrunken--------149
• Colorless full-----------152
• The first two reflects the majority classes of parental type as
paternal and maternal types or vice versa . This type of test is
an indication of linkage. As a ruler, the parental types have
much higher frequencies than the recombinant types. In fact,
this relationship can be used as a safe guide to identify the two
types in the test cross data whenever the identity of the
parental types is unknown to us.

18. LINKAGE MAPS & LINKAGE GROUPS
The linked genes together form a linkage group and the group
may be exhibited on a single straight line as the genes are to be
located along the straight line of the entire length of the
chromosome. The distance between the two neighboring genes is
directly proportional to the frequency of the recombination
percentage (%) between them. Such a line drawing showing the
order of the linked genes based on the recombination frequencies
is known as linkage map or genetic map or chromosome map. The
recombination between linked genes are determined from test
cross. The cross over % is used as a map unit. A map unit is the
distance in a chromosome which permits one % recombination
between two linked genes. A map unit is also called one Morgan (
1 centimorgan= 0.1 map unit). The map unit is an imaginary
distance and is not the mirror image of the actual distance
between two linked genes in the chromosome.

19. The gamete chromosome number (n) of a species reflects the
number of different linkage group in it. A linkage may provide
information on the genes that belong to the linkage group and the
expected frequencies of recombination between them. The total
map distance between the two genes of a linkage group may
exceed 50% or even 100% but it does not mean that they would
show more than 50% recombination. The frequency of
recombination between two linked genes can not exceed 50%
which is the frequency in case of independent assortment. There
is a 1:1 correspondence between map distance and the observed
recombination frequency up to 20 map units. But there is a
progressive decline in the frequency of observed recombination
for every additional map unit distance beyond 20 map units. A
map distance around 90% units is expected to show close to 50%
recombination.

21. 1. A map distance between two genes is equal to the frequency of
the recombination up to 20 map units.
2. Linked genes would show independent segregation if they are
more than 80 map units apart,
3. The maximum recombination observed between linked genes
is 50%.
The two genes belonging to the same linkage groups are called
syntenic .Such genes may show linkage or independent
seggregation depending on the distance between them.
Genetic linkage maps can be used to identify the location of
genes responsible for traits and diseases. Human genetic
linkage maps are important for two reasons. First, genetic
linkage maps can be used as a tool in linkage analysis,
association studies, and the building of physical maps.

22. IMPORTANCE OF LINKAGE
• Significance of linkage:
• Linkage does not permit the breeders to bring the desirable
characters in one variety. For this reason, plant and animal
breeders find it difficult to combine various characters.
• Linkage reduces the chance of recombination of genes and
thus, helps to hold parental characteristics together. It thus
helps organism to maintain its parental, racial and other
characters. Linkage has great impact on the genetic
correlation.
• The linked characters generally show high values of genetic
correlation and also of co-heritability. A linkage between genes
controlling two different desirable characters will help in
simultaneous improvement of both characters. If there is no
linkage between two desirable characters, the breeder has to
combine such characters from two different sources.

23. References:
1. Google for images,
2. Principles of Genetics- Basu & Hossain,
3. A textbook of Botany (Vol III) Ghosh,
Bhattacharya, Hait
4. Fundamentals of Genetics- B.D. Singh,
5.A Textbook of Genetics- Ajoy Paul
DISCLAIMER:
This presentation has been made to enrich open
source of information without any financial interest.
The presenter acknowledges Google for images and
other open sources of knowledge to develop this PPT.