Jill (mother) is heterozygous for gene A i.e Jill = Aa
Jack (father) who is homozygous recessive for gene A i.e Jack = aa
Now if both cross and fertilization occurs, we have:
a Aa aa
a Aa aa
The offsprings are (Aa, Aa, aa, aa)
Nondsijunction is the inability of homologous chromosomes to seperate properly during cell division. Now, we were told that this fertilization undergone nondisjunction in meiosis 1. Hence, both homologous chromosome move into the same daughter cells instead of different daughter cells because separation of homologous chromosome does not occur properly.
Therefore the result of these fertilization of a normal haploid sperm with an egg that had undergone nondisjunction in meiosis yields offspring with genotype Aa of their Mother (Jill).