Define complete linkage. Give an example of a cross showing complete linkage.

Explanation

Complete linkage : Linkage of genes on a chromosome which is not altered and is inherited as such from generation to generation without any cross over is called complete linkage. In this type of linkage, genes are closely associated and tend to remain together. They are transmitted together in their parental combination to the same gamete. Example, male Drosophila and female Silk worm (Bombyx mori). (a) Genes for grey body and vestigial wings in male drosophila. (b) Cross between red-eyed and normal imaged female drosophila.

Only parental combinations are formed in `F_2` generation and no appearance of any new combi-nations. 100% Parental combinations indicated that the gene for grey body colour is completely linked with long wings. In this dihybrid, `F_2` phenotypic ratio is 3 : 1 and test cross ratio is 1:1 (like a monohy-brid).