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MCAT Question A Day - 5/2/13 - Answer!

If, in one of two secondary spermatocytes, one of the 23 replicated chromosomes does not separate properly (nondisjunction) during Telophase II of meiosis, how many chromosomes will be present in the four spermatids produced at the end of the second meiotic division?

A.  23, 23, 22, 24
B.  23, 23, 23, 23
C.  22, 22, 24, 24
D.  0, 0, 46, 46

The correct answer is (A). Because one secondary spermatocyte (containing 23 replicated chromosomes or 46 chromatids) proceeds normally, it will produce two normal spermatids (23 chromosomes each) after the second meiotic division. The other secondary spermatocyte will have nondisjunction occur in one (2 chromatids) of the 23 replicated chromosomes, while the other 22 replicated chromosomes (44 chromatids) will separate normally. Therefore, one of these two spermatids will contain 22 chromosomes (it received neither of the still-attached chromatids), while the other will have 24 chromosomes (it received both of the still-attached chromatids).