Non-disjunction problem . Please explain every bit im truly trying to understand

Question

Non-disjunction problem . Please explain every bit im truly trying to understand this question i know the answer i just dont know how to properly solve this question

A mother has the genotype AaXX and a father has the genotype aaXY. Assume only one nondisjunction event for each part of this question.

A) If the couple has a child that is AAaXX. The nondisjunction event occurred in in meiosis 1 or two and is it in the father or mother ??

B)If the couple has a child with the genotype aaaXX, the nondisjunction event occurred in....

Explanation / Answer

A) The nondisjunction event occured in meiosis II and in the mother. Remember in meiosis I the homologous chromosomes become bivalents and the genetic make up would be A/a and X/X in mother. There would be two copies of each allele. Thus, nondisjunction of bivalents would give rise to combinations containing either AA or aa. In this case the combination would be AA (along with the single X chromosome) which fuses with another male gamete aX to give rise to the genotype AAaXX.

B) The nondisjunction event occured in meiosis I and in father. Here the bivalents fail to separate after meiosisI which was supposed to take place in meiosis II. Hence "aa" from the male fail to separate and fuse with "aX" from female and give rise to aaaXX genotype.

"A malaria biologist"

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