Question

If Jack and Jill have a child with an AAa genotype, during which meiotic division, and in which parent, could nondisjunction have occurred?

Answer 1

Further information from another source:

Jill is heterozygous for gene A and is going to have a child with Jack, who is homozygous recessive for gene A.

Answer:

Maternal meiosis II

Explanation:

Jill has the genotype Aa, and Jack has the genotype aa. Jack can only contribute the a, whereas Jill can contribute A or a. For the child to have 2 copies of the A allele and two copies of the a allele, that means the nondisjunction must have happened in the mother.

As for the stage of meiosis, non-disjunction in meiosis I means that homologous chromosomes fail to separate properly. This would mean that the child would inherit Aa from its mother and a from its father. This is not the case.

Non-disjunction in meiosis II means identical sister chromatids fail to separate properly, which means the child would inherit either aa from its mother, or AA from its mother, and a from its father. This could give the genotype AAa. Therefore, nondisjunction must have occurred in maternal meiosis II