Question

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

Answer 1

Complete linkage:

1. When genes are closely located in chromosomes and there is no chance of separating them by even crossing over, then it is called complete linkage.
2. They are transmitted together to the same gamete and offspring.
3. Following are the factors that affect linkage:
   • Distance between the genes: As the distance between genes increases, the chance of linkage decreases.
   • Age: With the increase in age, the chances of crossing over generally decrease which increases linkage.
   • Temperature: A rise in the temperature causes the chances of chiasmata formation.
   • It decreases the strength of linkage.
   • X-rays: The strength of linkage decreases if the genes are exposed to X-rays.
4. For example, male Drosophila and female silkworm(Bombyx mori).
5. 100% Parental combinations indicated that the gene for grey body colour is completely linked with long wings.
6. In this dihybrid, the F2 phenotypic ratio is 3:1and the test cross ratio is 1:1(like a monohybrid).
7. When a female wild fly with a black body and vestiges of wings crossed with an ordinary male wild fly with a grey body and regular wings, the resulting F1 hybrids were all grey bodied and normal-winged (with dominant characters).
8. However, only two types of individuals rather than the anticipated four were produced in the F2 generation when F1, a male, was backcrossed with a recessive female parent.
9. These two types were equally distributed among grey-bodied and normal winged and black-bodied and vestigial winged individuals, demonstrating the full relationship.