Question

Two normal parents have a son with hemophilia (an X-linked recessive disorder)

that also has Klinefelter Syndrome.

a. Write out the cross as described above illustrating the genotypes and

phenotypes of the parents and child.

b. Did the non-disjunction event occur in the mother, the father, or could

have occurred in either of the two parents?

c. Did the nondisjunction event occur during meiosis I, meiosis II, or could it

have occurred in either division during meiosis.

Answer 1

Klinefelter syndrome occurs when the male child receives an extra copy of X chromosome thereby making him XXY.

Klinefelter syndrome is formed as a result of nondisjunction during meiosis in either of the parents. The nondisjunction results in uneven distribution of chromosomes and thus some gametes that are formed have two copies of the sex chromosome instead of one.

Here the offspring also has hemophilia which is an X-linked recessive disorder, while both the parents are normal. Hence the two X chromosomes in the son are Xh or recessive. Since the father is normal, he will not have Xh. Thus, the non-disjunction event has occurred in the mother. Hence the female is the carrier for hemophilia and is XhX while the male parent is XY and the offspring produced got Y from father and two copies of Xh from the mother.

Now the nondisjunction event could have occurred in either Meiosis I or Meiosis II. If it had occurred in Meiosis II, then the offspring would have received one copy of X and the other of Xh. However, since he received both copies of Xh, the nondisjunction occurred in Meiosis I.