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"Consider a pair of homologous chromosomes heterozygous for three genes (e.g., ABC/abc) during prophase I of meiosis. Let the sister chromatids of one homolog be numbered 1 and 2 (ABC); and the sister chromatids of the other homolog be numbered 3 and 4 (abc).

"Assume a double crossover occurs in this pair of chromosomes that results in chromatids of the genotype Abc, ABC, aBc and abC. If the first crossover (the one between A and B) involves chromatids 1 & 4, which chromatids could be involved in the second crossover?"

A. 1 & 2
B. 1 & 3
C. 1 & 4
D. 1 & 3 or 1 & 4 or 2 & 3 or 2 & 4
E. any two of the four chromatids

I'm a little confused about how to set up this problem. When the problem says "sister chromatids...1 and 2 (ABC)" and "sister chromatids...3 and 4 (abc)" does that mean that ABC are on both chromatids 1 and 2, and abc are on both chromatids 3 and 4? I.e.,

1 A----B----C
2 A----B----C
3 a----b----c
4 a----c----c

If not, I'm not sure which alleles are on which chromatids.

Also, I initially thought that answers A and E could not be correct because I thought that only nonsister chromatids could cross over. But if that's true, how does a double crossover between 1 and 3 or 1 and 4 produce the genotype ABC? Couldn't that only be obtained with a double crossover between 1 and 2? Or does a crossover + and reverse crossover count as a double crossover? I.e.,

1 A---B---C
3 a---b---c

crosses once between A and B to produce A---b---c, and once more between A and B to produce A---B---C?

As you can see, I'm a little confused. Can you please describe how to do this problem, and include the answer so that I can make sure I understand your explanation when I try to problem on my own?

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