Linkage and mechanism of linkage

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The tendency of genes or characters to be inherited together because of their location on the same chromosome is called linkage.

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.

It states that the inheritance of genes of each pair in a dihybrid during gamete formation is independent of the other.

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) dominant over round pollen (l).

They crossed true breeding plants having blue flower 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 Fl hybrid and double recessive parental stock red round (bbll) did not result in ratio 1:1:1:1 but gave unexpected phenotype frequency as shown below.

Here, blue long and red round are parental forms and show greater frequency 88 per cent.

Blue round and red long are recombinant forms and show lesser frequency 12 per cent. 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 flower with round pollen (BBll) and red flower with long pollen (bbLL). A testcross between

heterozygous blue long (BbLl) of Fl hybrid and double recessive red round (bbll) did not result in ratio 1:1:1:1 but gave unexpected phenotype frequency as shown below.

Here, blue round and red long are parental forms and show greater frequency 88 per cent.

Blue long and red round are recombinant forms and show lesser frequency 12 per cent.

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 explanation for 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 of their tendency 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 result because the genes are linked

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Other links

STRUCTURE OF CHROMOSOME – CELL BIOLOGY

Types of chromosomes with special types

Gene and genome

Crossing over, gene mapping and recombination of chromosome

Mutation and classification of mutation

Mutagenic agents and its significance