Question

Drosophila melanogaster normally have four sets of chromosomes. What total number of chromosomes (actually the total number of chromatids is a more accurate way to phrase this) will the four gamete cells have after a single non-disjunction event in MII? Write four numbers separated by commas

Answer 1

Chromosomes are double armed structures where each arm contains two sister chromatids. Thus, a chromosome contains 2 sister chromatids. These chromatids separate from the centromere and split into two parts. Each part of the sister chromatid is passed on to the gametes so that crossing over and recombination might take place during meiosis II.

Ideally for Drosophila melanogaster, there are 4 chromosomes in doublet or two copies i.e. 8 chromosoems total. Also, 8 chromosomes will have 8*2=16 chromatids. Thus, each gamete will contain 4 chromosomes but not paired. Thus, each gamete will have 4*2 = 8 chromatids after a single non-disjunctioning event in meiosis II.

However, if non-disjunctioning does not take place the four pairs of chromosomes will be segregated as:

1 chromosome complete, 2 chromatids each splitted from 1 chromosome, 2 chromatids each splitted from 1 chromosome, 2 chromatids each splitted from 1 chromosome.