1. A male with an extra X chromosome (XXY) has a disorder called Klinefelter syndrome and cannot reproduce. How could this disorder happen? a. Having an extra chromosome or a missing chromosome is called aneuploidy , and it often results from nondisjunction : an error in meiosis in which a pair of homologous chromosomes fails to separate in meiosis I, or a pair of sister chromatids fails to separate in meiosis II.
Suppose nondisjunction for the sex chromosomes occurred in this man’s mother during meiosis
I, but sister chromatids separated normally in meiosis II. Show the two cells that would result after meiosis I and the four cells that would result after meiosis II. You only need to show the sex chromosomes, not any of the others.
The mother has two X chromosomes
(XX). Normally, the two cells produced by meiosis I should each get one (X).
However, if there is nondisjunction, one cell would get both (XX) and the other no sex chromosome (O).
After meiosis II, the sister chromosomes would have separated, but the daughters of the first cell would still have two X chromosomes (XX), and the daughters of the second cell still no sex chromosome.
b. What would happen when these possible egg cells are fertilized? List all the possible sex chromosome combinations for the offspring. Could this have been the cause of his Klinefelter syndrome?
The father can give either X or Y; the mother’s gametes are either XX or have no sex chromosome (O). So,
XX + X = XXX
XX + Y = XXY – this would result in Klinefelter syndrome
O + X = XO (this one is Turner syndrome)
O + Y = YO (dies as an embryo) c. Could the XXY male have resulted from nondisjunction during meiosis II in his father? Explain.
Meiosis I in the father separates X from Y; nondisjunction would then produce sperm cells that are XX, YY or O. The mother can only contribute X, so the possible offspring are XXX, XYY and XO. Klinefelter syndrome cannot result from this event.
2. A female with three X chromosomes (XXX) is anuploid but is just as healthy as a chromosomally normal woman. Circle all the possible causes of this example of aneuploidy below:

a. Nondisjunction in the mother during meiosis I b. Nondisjunction in the mother during meiosis II c. Nondisjunction in the father during meiosis I d. Nondisjunction in the father during meiosis II
Nondisjunction at either division in the mother can produce XX, fertilization with an X-bearing sperm then produces XXX. Nondisjunction during meiosis II in the father can produce XXX as in
1c above. However, nondisjunction in the father during meiosis I gives an XY cell and an O cell, resulting in XXY or XO offspring.