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Question: Given diagram shows a pair of homologous chromosomes during meiosis. Maximum crossing over will occ...

Given diagram shows a pair of homologous chromosomes during meiosis.

Maximum crossing over will occur between genes

A

A and a, D, and d

B

C and d, c and D

C

B and c, b and C

D

A and d, a and D

Answer

d

Explanation

Solution

Crossing over is the exchange of genetic material between non-sister chromatids of homologous chromosomes during meiosis. The frequency of crossing over between two genes is directly proportional to the physical distance between them on the chromosome. Genes that are farther apart have a higher probability of undergoing crossing over than genes that are closer together.

  1. Identify the genes and their arrangement: The diagram shows two homologous chromosomes with genes arranged linearly as A-B-C-D on one chromosome and a-b-c-d on the homologous chromosome.

  2. Understand the concept of genetic distance: The "distance" between genes in the context of crossing over refers to the physical distance along the chromosome. We can assume that the segments between adjacent genes (A-B, B-C, C-D) represent equal units of distance.

  3. Evaluate the distance for each option:

    • (a) A and a, D and d: These represent alleles at the same gene locus (e.g., A and a are alleles of gene A). Crossing over occurs between different gene loci, not within the same locus. Therefore, this option is incorrect.
    • (b) C and d, c and D: This implies considering the distance between the gene locus C and the gene locus D. Looking at the diagram, C and D are adjacent genes. Let's say the distance between adjacent genes is 1 unit. So, the distance between C and D is 1 unit.
    • (c) B and c, b and C: This implies considering the distance between the gene locus B and the gene locus C. B and C are also adjacent genes. So, the distance between B and C is 1 unit.
    • (d) A and d, a and D: This implies considering the distance between the gene locus A and the gene locus D. To go from A to D, we must pass through B and C (A → B → C → D). The total distance is the sum of distances A-B, B-C, and C-D. If each adjacent segment is 1 unit, then the distance between A and D is 1 + 1 + 1 = 3 units.

  4. Compare the distances:

    • Distance for (b) = 1 unit
    • Distance for (c) = 1 unit
    • Distance for (d) = 3 units

Since the distance between A and D (and their alleles a and d on the homologous chromosome) is the largest (3 units), the probability of crossing over occurring between these genes will be maximum.