Question

The double crossover type of combinations of two chromatids are furnished below. Which one of these gives no recombination?
A. Four strand double crossover.
B. Two strand double crossover.
C. Three strand double crossover.
D. Another three strand double crossover.

Answer 1

Hint: In recombination, chromosome segments are exchanged between two non-sister chromatids in meiosis. Crossing over results in a new combination of genes that are absent in either of the parents.

Complete step by step answer:
Let’s discuss the options and find out the correct answer.
- If crossover occurs at the fourth strand stage, after the replication there are four chromatids with sister chromatids being unlike to each other. The homologous chromosomes that had been sampled from the same meiosis may or may not be of reciprocal types. Hence, this is the correct answer.
- If crossover occurs at the two-strand stage, homologs cross over earlier than replications. After the replications are done, the sister chromatids produced are always alike. If two non-homologous chromosomes are sampled from the same meiosis would be reciprocal types provided replications are done after the recombinations.
- In three strand double crossover case also, replications are done after crossing over just as in two strand crossovers. The results will also remain the same.
- Another three strand double crossover is not at all possible.
Hence, option ‘A: Four strand double crossover’ is the correct answer.

Note: Crossover was discovered by Thomas Hunt Morgan. Crossover is also called genetic recombination. Crossover also helps in exchange of genetic materials. In the process of meiosis, the homologous chromosomes get separated to form gametes.