Question

nondisjunction of the sex chromosomes during meiosis 2 in human males during gametogenesis can result in all of the following complements in a gametes EXCEPT

a. no sex chromosomes

b. XX

c. XY

d. YY

e. X

Answer 1

The correct answer should be (c) XY.

Reason : Before disjunction occurs in Meiosis 2, the sex chromosomes are in the form of XX and YY. When disjunction will occur, they'll form 2 X and 2 Y type sperms each.

Due to non-disjunction, if XX and YY are unable to separate, then there are only four possible outcomes :

i) If disjunction occurs in YY only : The resultant gametes will be XX, Y and Y.

ii) If disjunction occurs in XX only : The resultant gametes will be YY, X and X.

iii) If no disjunction occurs in both XX and YY : The resultant gametes will be XX and YY.

iv) If disjunction occurs in both XX and YY : The resultant gametes will be X, X, Y and Y. (Which actually happens)

So out of the options provided in the question, only XY is the case which is not possible. Initially X and Y are not in form of sister chromatids, hence their recombination during meiosis 2 in spermatogenesis itself is highly unlikely. The formation of XY is possible only when non-disjunction occurs during meiosis 1 only.

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