Linkage and crossing over

 The tendency of two or more genes or traits to inherit together is called linkage. During the process of meiosis, genes that are present on the same chromosome tend to be inherited together as a unit, called linkage, but in some cases, alleles of the linked genes recombine by crossing over and the linkage is broken. Linked genes thus rarely ever assort independently unlike genes present on separate chromosomes. Linked genes travel together in the same gamete as they are not independent of each other. The behavior of such kind of genes are said to deviate from

Mendel’s law of independent assortment.

In the Mendelian experiment, the common sweet peas with two or more “elements” (now called genes) are considered heterozygous. In a hybrid, the segregation of anyone is completely independent of the rest. That means, all the genes are distributed to the gametes at random. Mendel considered the inheritance of seven characters and all the characters showed random assortment. It was later demonstrated cytologically that peas have seven pairs of chromosomes.

It was genetically shown that the genes for the characters studied by Mendel are each located in a different member of the seven pairs of chromosomes. The independent assortment between two gene pairs, each with complete dominance and affecting different characters, results in a 9: 3: 3: 1 ratio among the offspring of heterozygotes for both pairs (AaBb x AaBb).

Many hybridization experiments were conducted both on plants and animals based on Mendel's work. The results of certain dihybrid crosses did not confirm the law of independent assortment. In 1906, William Bateson and Reginald Punnett conducted experiments in sweet pea, Lathyrus odoratus to confirm Mendel's dihybrid testcross. They observed an exception to the independent assortment of two genes in this plant. Here, blue flower (B) is dominant over the red flower (b), and long pollen (L) is dominant over round pollen (l). They crossed true-breeding plants having blue flowers with long pollen (BBLL) and red flower with round pollen (bbll). All the F1 hybrids have blue flowers with long pollen (BbLl). A testcross between heterozygous blue long (BbLl) of F1 hybrid and double recessive parental stock red round (bbll) did not result in ratio 1:1:1:1 but gave unexpected phenotype frequency.

Here, blue long and red round are parental forms and show a greater frequency of 88 percent. Blue round and red long are recombinant forms and show a lesser frequency of 12 percent.

The dihybrid test cross ratio obtained is 7:1:1:7 and not 1:1:1:1. This indicates that the genes do not independently assort. From the above test cross, it is clear that if dominant alleles or recessive alleles are present in the same plant, they tend to remain together resulting in increased parental forms. Thus, the two genes which inherit together are called linked genes. This aspect is called coupling.

They made another cross between plants having blue flowers with round pollen (BBll) and red flower with long pollen (bbLL). A testcross between heterozygous blue long (BbLl) of F1 hybrid and double recessive red round (bbll) did not result in ratio 1:1:1:1 but gave unexpected phenotype frequency.

Here, blue round and red long are parental forms and show a greater frequency of 88 percent. Blue long and red round is recombinant forms and shows a lesser frequency of 12 percent.

The dihybrid test cross ratio obtained is 1:7:7:1 and not 1:1:1:1. This indicates that the genes do not independently assort. From the above testcross, it is clear that if dominant alleles or recessive alleles are present in the different plants, they tend to remain separate resulting in increased parental forms. This aspect is called repulsion.

Coupling and repulsion offered explanations for a higher frequency of parental forms. They are two aspects of a single phenomenon called linkage. The genes that are carried on the same chromosome will not assort independently because they tended to remain linked together. This is called linkage. The genes located on the same chromosomes that are inherited together are known as linked genes. They tried to reconfirm the law of independent assortment. But they could not get expected results because the genes are linked.