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Answer to Question #72496 in Genetics for Sam

Question #72496
I have a question regarding non-disjunction that I'm struggling with. The question is "Normally a make of genotype produce sperms with genotype(s) A or a. Occasionally non-disjunction happens during meiosis 1 and consequently results in sperm with genotypes of (pick all that apply):
1) AA
2) Aa
3) aa

Similarly, if this non-disjunction error occurred in meiosis 2, what genotypes would the sperm have?
Expert's answer
Solution

Regarding the fact that organism can produce both A- and a- containing sperm cells, we may conclude that the genotype is Aa – the organism is heterozygous.

If non-disjunction happens during the first meiotic division, than cells that enter the second meiotic division have either genotype AAaa or no these chromosomes at all. The latter is not important in this task, but in the former cells tetrads can be separated in any way. So each of the proposed variants (AA, Aa and aa) can be formed.

If non-disjunction happens after the first meiotic division, this means that cells that enter the second division contain only one doubled chromosome out of the pair. Therefore, they can be AA or aa only. Therefore, after the second division we can obtain gametes AA and aa but not Aa.

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