Explain two types of linkage.

Explanation

Linkage. The phenonenon of inheritance of genes together due to their occurrence on the same chromosome is called linkage. Linked genes.occur on same chromosome while unlinked genes occur in different chromosomes. Linked genes do not show independent assortment. They do not give a 9:3:3:1 dihybrid phenotypic ratio and 1:1:1:1 test cross ratio. Linkage may be : (i) Complete, (ii) Incomplete. 1. Complete Linkage (Morgan, 1919). The genes located in the same chromsome do not separate and are inherited together over the generations due to the absence of crossing over. Complete linkage allows the combination of parental traits to be inherited as such. It is rare but has been reported in male Drosophila and some other heterogametic individuals. e.g., A red eyes normal winged (wile type) pure breeding female Drosophila is crossed to homozygous recessive purple eyed and vestigial winged male. The progeny or F, generation individuals are heterozygous red eyed and normal winged. When F, male are test crossed to homozygous recessive female (purple eyed and vestigial winged), only two types of individuals are produced-red eyed normal winged and purple eye vestigial winged in the ratio of 1:1 (Parental phenotypes only). Similarly inbreeding of F, recombinant types are absent. In practice, this 1 : 1 test ratio is never achieved because total linkage is rare. 2. Incomplete Linkage. 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 in more than 25% while each of the recombinant types is less than 25%. e.g., In Drosophila, crossing of yellow bodied (Y) and white eyed (W) female with brown bodied (Y) red eyed (W*) male produced F, to be brown bodied red eyed. On intercrossing of F, progeny, Morgan observed that the two genes did not segregate independently of each other and therefore, the F, reatio deviated significantly from expected 9:3:3:1 ratio. He found 98.7% to be parental and 1.3% were recombinants.