Question

these homologous chromosomes carry different alleles of the a and b genes. how could crossing over affect the distribution of the four alleles: a, a, b, and b? a. crossing over would always keep a and b together on the same chromosome, and keep a and b together on the same chromosome. b. crossing over would always put a and b on different chromosomes, and always put a and b on different chromosomes. c. crossing over would sometimes, but not always, exchange the positions of a and a, and of b and b, on homologous chromosomes. d. crossing over would sometimes, but not always, delete one or more of the four alleles from both homologous chromosomes.

Explanation:

Crossing - over occurs during meiosis. It involves the exchange of genetic material (alleles) between homologous chromosomes. It is a random process. It does not always happen (so it is "sometimes"), and when it occurs, it can exchange the positions of alleles (like \(A\) and \(a\), \(B\) and \(b\)) on homologous chromosomes. Option A is incorrect because crossing - over can separate \(A\) and \(B\). Option B is incorrect as it is not "always" that \(A\) and \(B\) are on different chromosomes. Option D is incorrect because crossing - over is an exchange, not a deletion of alleles.

Answer:

C. Crossing over would sometimes, but not always, exchange the positions of \(A\) and \(a\), and of \(B\) and \(b\), on homologous chromosomes.