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Please consider the following question:

Genes A and B are located on the same chromosome. If two individuals with the genotype shown below are crossed, which genotype could be formed? enter image description here Let / represent the line in the image above. a.)AB/ab b.)Ab/Ab c.)aB/aB d.)AB/Ab

Why can't the answer to this question be all of the choices? I thought that recombination was completely randomized through the process of crossing over. Why is only one of the choices correct and how would I go about doing this on my own?

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The answer is for Genes A and B on the same chromosome is a. AB/ab – user19978 Nov 13 '15 at 1:33

One of the alleles for each gene must come from the parent on the right hand side. That parent only has a and b alleles (is homozygous) - meiotic crossing over will not change this. So the offspring must inherit at least one a and one b. That should point you to the answer.

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