Linkage and Crossing Over (Class XI/XII)

Alleles coming from the same parent tend to enter the same gamete and to be inherited together, is called coupling.

In Punnet's experiment, 7 : 1 : 1 : 7 ratio in F2 generation was due to coupling and repulsion.

The genes located on the same chromosome that are inherited together are called linked genes.

Recombination of genes takes place at pachytene stage. During pachytene stage of meiosis I, non-sister adjacent chromatids in a tetrad, coil around each other at one or more points called Chiasmata. Chromatids break down at Chiasmata and then rejoin with the exchange of segments resulting in recombinants.

The strength of linkage depends upon the distance between the genes. Distance between the genes is inversely proportional to strength, i.e. D is proportional to 1/S, where D is the distance and S is the strength of linkage.

Complete linkage is observed in Drosophila males. In complete linkage, the genes located on the same chromosome do not separate and are inherited together over the generations due to the absence of crossing over.

The ratio of parental recombination in maize in Hutchinson's test cross is 96.4 : 3.6.

Synaptonemal complex is a molecular scaffold that aligns the DNA molecules of two chromosomes side by side during synapsis. As a result, DNA strand of one chromosome pairs with the corresponding DNA strand of the other.

Genes present in the same chromosome have a tendency to separate due to crossing over and hence produce recombinant progeny besides the parental type. The number of recombinant individuals is usually less than the number expected in independent assortment. In independent assortment, all the four types (two parental types and two recombinant types) are each 25%. In case of linkage, each of the two parental types is more than 25% while each of the recombinant types is less than 25%.

Genetic repulsion is defined as, the genes or alleles coming from two different parents tend to enter different gametes and to be inherited independently.

There are 23 linkage groups in humans. The total number of linkage groups is equal to the number of homologous pairs of chromosomes.

There are 10 linkage groups present in maize.

The principle of linkage was first observed in Pea by Bateson and Punnet in 1906.

If genes are linked, segregation of alleles could be prevented. The linked gene tend to remain together on the same chromosome.

Mendel didn't notice linkage due to independent assortment. He unknowingly chose those characters which are either on different chromosomes or lay apart on the same chromosome. The characters were assorted independently in F2 generation as they were not linked.