The cross of an aa male and Aa female produced an offspring

The cross of an aa male and Aa female produced an offspring with the genotype AAa. Therefore, the resulting offspring has 3 copies of gene 1 (trisomy for Chromosome 1). At what stage during meiosis did the non-disjunction event occur and in which parent?

Circle all of the answers that apply.

a) Paternal non-disjunction in Meiosis I

b) Maternal non-disjunction in Meiosis I

c) Paternal non-disjunction in Meiosis II

d) Maternal non-disjunction in Meiosis II

Solution

Ans. Given,

Male parent genotype = aa

Female parent genotype = Aa

Genotype of the offspring (trisomy) = AAa

Part 1: Presence of two “A” chromosomes indicates that the non-disjunction of chromosomes during gametogenesis occurred in the female parent because she only has “A” chromosome.

Thus, the disjunction in – maternal.

Part 2: During S-phase of meiosis, cell duplicates its chromosome. So, the cells karyotype (maternal gametogenesis) at after completion of S-phase is “AAaa”. A set of A and a chromosome is called homologous pair ; whereas each ‘AA pair or aa’ pair is called sister chromatids. That is, the cells have 2 pairs of sister chromatids- one AA and one aa – and both pairs (AA-aa) are also linked together (but weakly than A-A or a-a pairing).    

Meiotic disjunction occurs in two phases-

1. Meiosis I disjunction: Homologous chromates separate. Thus, the maternal cell AAaa divides into one ‘AA’ and the other ‘aa’ daughter cells.

2. Meiosis II disjunction: Sister chromates separates into two independent chromosomes. That AA cell forms two cells each with chromosome ‘A’ ; similarly ‘aa’ cell forms two cells each with chromosome ‘a’.

Thus, for a female gamete to have two A chromosomes, there must have occurred non-disjunction during meiosis II.

Thus, the disjunction in – meiosis II.

Therefore,

The correct answer is- d. Maternal non-disjunction in meiosis II.