Linkage:-
LINKAGE is the tendency of two or more genes to live together in the same chromosome during the genetic transmission.
Introduction:-
Large no. Of genes are found in each species of plants to produce various characters.
The no. Of genes is from thousands to lakhs while the no. Of chromosomes are constant and limited.
The no. Of chromosomes more than 100 is rarely found. Hence, obviously numerous genes should be present on single chromosome.
Since, Chromosomes reach to the gametes by meiosis i.e. chromosomes acts as a carrier of genes to next generation.
Definition:- such genes which are situated on one chromosome only and are always inherited together in coming generation are called linked genes and this phenomenon is called linkage.
Such characters whose genes are present on same chromosome the law of independent assortment is not applicable.
This fact is firstly reported by BATSON and PUNETT when they failed to observe the independent assortment in two allelic pairs of sweet pea.
They reported that the results obtained in F-2 generation by performing two dihybrid crosses for colour of flower and length of pollen grain were totally different from the law of independent assortment.
The phenotypic ratio expected was 9:3:3:1 but here the phenotypic ratio of offsprings of dihybrid test cross of such type of character were 1:7:7:1.
BATSON and PUNNETT reported that the above dihybrid crosses are due to coupling and repulsion tendencies of both the characters of genes.
They explained that when two dominant genes come together from one parent only then they inherited into the coming generation together due to which the offspring bearing parental kind of characters are obtained in much larger no. And the offsprings bearing new kind of characters or recombinants are less in no. .
Coupling - This type of tendency of two dominant genes is called coupling.
Types of linkage:-
Linkage are of two types:-
1) Complete linkage- according to MORGAN- "Complete linkage is When recombinants are not formed in linked genes, that is those linked genes in which crossing over does not occur".
It is found among such genes which are situated very close to each other on the same chromosome due to which crossing over does not occur between them. This type of linkage occurs very rarely and it is studied for only some characters of male drosophila .
Example:- MORGAN (1911) crossed an ordinary wild type drosophila with grey body long wings (BBVV) with another drosophila (mutant type) with black body and vestigeal wings (bbvv).
All the hybrid of F-1 generation are with grey body and long wings (BbVv). I.e. phenotypically like the wild type of parents.
now a male of F-1 generation having genotype (BbVv) is back crossed with double recessive female (test cross) with black body and vestigeal wings having genotype (bbvv) then only parental combinations are formed in F-2 generation and no appearance of any new combinations.
The results of above crosses indicate that grey body character is inherited together with long wings. It implies that these genes are linked together. Similarly, black body character is inherited together with vestigeal wings.
Since only parental combination of
Character will appear in F-2 generation and no new or non parental combination appears.
This shows complete linkage.
Incomplete linkage:-
When some recombinants are formed in linked genes i.e. some crossing over occurs between linked genes, it is called incomplete linkage.
This type of linkage is found among such genes that are situated far apart on the same chromosome and expected to get separated from each other during crossing over.
Such type of genes are called incompletely linked genes and this condition of their inheritance is called incomplete linkage.
Incomplete linkage is very common and has been studied in almost all the organism.
Example:-
HUTCHINSON crossed a variety of maize having homozygous coloured and full seeds (CCSS) with a variety having heterozygous colourless and shrunken seeds (ccss) .
All the F-1 plants produced coloured and full seeds ( CcSs).
But in a test cross, when such F-1 females (CcSc) heterozygous are cross pollinated with the pollen from the plant having colourless and shrunken seeds (ccss) double recessive, four types of seeds are produced.
1) coloured and full seeds
2) colourless and shrunken seeds
3) coloured and shrunken seeds
4) colourless and full seeds
It is clear that, the parental combination are more in number (96.4%) then the new combinations (3.6%). These genes are located in same chromosome and only 3.6% individual these genes are separated by crossing over. This is an example of incomplete linkage.
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