Name: Sex Determination (Drosophila melanogaster)
Uploaded: 2019-07-19
Duration: 7 min 20 s
Description: Drosophila melanogaster or the common fruit fly has contributed a lot to genetics. One of them is its sex determination system with more than five types of s...

Question

"Complete linkage is rare" explain it with a suitable example.

Accepted Answer

Complete linkage is the tendency of genes present in the same chromosome to remain in their original combination and enter together without crossover into the same gamete. In 1910, Morgan announced "Chromosome Theory of Linkage" while working on Drosophila, claiming that coupling and repulsion are two aspects of the association. He defined complete linkage as the tendency of genes present in the same chromosome to remain in their original combination and enter together without crossover into the same gamete. Genes are closely linked in this form of association, and appear to stay together.

During meiosis, the attachment is broken down by the act of crossing over. If genes are so closely connected that they are still expressed together, they are not crossed over, the relation between them is called complete as illustrated in an example below:

Where a male Drosophila and female silkworm (Bombyx mori) have 100% parental combinations showed that the body colour gene is entirely correlated with long wings. The phenotypic F2 ratio in this dihybrid is 3:1, and the test cross-ratio is 1:1 (like a monohybrid).

In the case of male Drosophila recombinant forms are absent due to lack of crossing over. This means that in their original configuration all the genes that originate on a single chromosome will end up on the same chromosome. Only parental phenotypes are predicted, in the absence of recombination.

"Complete linkage is rare" explain it with a suitable example.

Important Questions on Principles of Inheritance and Variation

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