Law of Independent assortment suggest that an individual with AaBb genotype will produce four types of gametes viz. AB, Ab, aB and ab in equal proportion (1:1:1:1) and a di-hybrid ratio of 9 : 3 : 3 : 1 and 1:1:1:1 test ratio would be obtained.
However when few or more number of genes are located on same chromosome, independent assortment will not take place.
Bateson and Punnett (1906) described a cross in sweet pea, where independent assortment was not observed.
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They crossed a variety having blue flower ‘B’ with long pollen ‘L’ and another variety with red flowers ‘b’ and round pollens ‘1’, F, individuals had BbL1 i.e. blue flowers with long pollens.
When these plants were crossed with red flowers having round pollens bb11, they observed 7: 1: 1: 7 ratio instead of 1: 1 : 1 : 1 indicating that there is tendency in dominant alleles to remain together. Similar was the case with recessive alleles.
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The tendency of genes to stay together during inheritance due to the genes being located relatively close to each other in the same chromosome, produces typical distortion of test cross ratios.
1. Coupling phase:
In linkage, the dominant alleles of two or more genes present in the same chromosome and hence linked together. Dominant alleles are contributed by the same parents.
2. Repulsion phase:
A linkage between dominant alleles of one or more genes and the recessive alleles of another (several genes) in such a case one parent involved in cross contributes the dominant allele of one gene while second parents provides the dominant allele of other genes.
Bateson and Punnett explained the lack of independent assortment in above experiment by a hypothesis known as coupling and repulsion. This can be explained with following example.
The condition in which two dominant genes tended to enter the gametes together in greater number than random proportion is called coupling (BBLL x bb11) while the tendency for one dominant and one recessive gene to enter the gametes in the greater proportion is called repulsion (BB11 X bb11).
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Two other terms ‘cis’ (coupling) and ‘trans’ (repulsion) also have been used by organic chemistry people.
No satisfactory answer to this process of coupling and repulsion was given until Morgan’s (1910), experiments on Drosophila. He replaced the theory of coupling and repulsion with that of ‘linkage and crossing over’.
He found that coupling and repulsion was not complete. He proposed that two genes are found in coupling phase or in repulsion phase because these genes were present on same chromosome (coupling) or as two different chromosomes but homologous chromosomes (repulsion).
Such genes are then called linked and phenomenon is called linkage. Morgan further suggested that coupling and repulsion are two aspects of single phenomenon called linkage.
The strength of linkage is determined by distance between two genes. Greater the distance lower will be the linkage strength.
Linkage is broken down due to the phenomenon of crossing over occur during meiosis. Crossing over will be relatively more frequent if distance between two genes is more.
The process of crossing over involves exchange of chromosome segments. Morgan’s concept about the linkage developed the theory of linear arrangement of genes in the chromosome which helped to the construction of genetic or linkage maps of chromosomes.
Linkage was defined as the tendency of the genes in the same chromosome or linkage group to enter the gametes in groups and pass on to the next generation.
Morgan supposed that this tendency of linked genes to remain in their original combination was due to their residence/location on the same chromosome.
While, crossing over is the tendency of genes to enter the gametes in combinations i.e. recombinants.