Chapter 7: Linkage
1 What is linkage? When does it occur? Which of Mendel's laws is violated if two genes are linked?
Ans:
When a number of genes are present on the same chromosomal arm, then in proportion to the distance among their loci, they resist recombining during crossing over. This is called linkage. Hence, the genes that are very close enough, then during crossing over, they will be transferred together during crossing over, while those that are far apart will be more prone to get separated.
2. How does crossing over break up linkage?
Ans:
Crossing over causes the exchange of a portion of the chromosome with that of a homologous chromosome and thus it causes separation of the genes situated on the same arm. In this way, it breaks up linkage.
3. What structures actually go through crossing over?
Ans:
DNA actually go through crossing over.
4. If a test cross is performed, what phenotype ratio would you see? Which of the phenotypes
would be recombinant and which would be non-recombinant? If linkage is suspected, how would
it change those numbers?
Ans:
If a test cross is performed, then the phenotypic ration would be 1:1. Any deviation from that indicates the appearance of linkage & crossing over. If the linkage is suspected, then there will be four types of phenotypes instead of two with the highest number of them indicate the parental types, while the lower two numbers indicate the recombinant type.
5. Why, if you performed a testcross, could the percentage of recombinant offspring not exceed
50%?
Ans: Because only one pair of the non-sister chromatids take part in crossing over. Hence, in a tetrad since there are only 4 chromatids and 2 out of them are taking part in crossing over, so the maximum chances of recombinants are only half that is 50%.

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