Which of the following pairs of alleles would be most likely to cross over?

() A ... B

() A ... B ...

() A ... B ... C

() AB ...

() ... AB

a) A ... B

Explanation:

Crossing over is the process that occurs during the pachytene stage of prophase-I of meiosis-I. During crossing over, two chromatids of a homologous pair of chromosomes exchange genetic material between themselves. It results in the production of new allele combinations on the chromatids. The process of crossing over mostly occurs between the alleles of two or more genes that are present far apart from each other on the same chromosome.

Among the given option, the arrangement of alleles in option A has the maximum distance between the alleles A and B. This increases the probability of crossing over between these two alleles. The alleles of genes present close together on a chromosome are least likely to undergo crossing over.

Option D and E, AB ... and ... AB

Explanation:

When the alleles are located far away from each other on the same chromosomes they remain unlinked and hence assort independently. On the other hand alleles which are lying close to each other on the same chromosomes are inherited as a single unit or are inherited together; therefore they are called as linked allele pair. Crossing occurs when the two chromosomes exchange their genetic material. When two genes lie very close to each other, there are very rare chances of them getting separated by crossing over.

Hence, option D and E are correct