a. People with the chromosome constitution 47, XXY have Klinefelter syndrome and

Question

a. People with the chromosome constitution 47, XXY have Klinefelter syndrome and are phenotypically male. A normal woman, whose father had the recessive X?linked disease, hemophilia (h = mutant, versus H = normal), mates with a normal man and they produce a son with Klinefelter syndrome who also has hemophilia. In which parent and in which stage of meiosis did nondisjunction occur (i.e., Meiosis I or Meiosis II)? Explain your answer.

b. People with the chromosome constitution of 46, X0 have Turner syndrome and are phenotypically female. A normal woman whose father had hemophilia, mates with a normal man and they produce a daughter with Turner syndrome who also has hemophilia. In which parent and in which stage of meiosis did non?disjunction occur? Explain your answer

Explanation / Answer

a.

Both the parents of the given individual are normal. Since the woman’s father is affected by the condition, she must be a carrier. The male parent may or may not be a carrier of the defective haemophilia allele (Xh).

The Y chromosome was inherited from the father. Both the X chromosomes are inherited from the mother/female parent. The nondisjunction occurred during meiosis II in the egg, where the sister chromatids of x and x failed to separate. An xhxh egg is produced, which when fertilized with the normal male gamete, gave the offspring xhxhY genotype.

There is only a single X chromosome in the male parent and any mutation would have caused either of the conditions in the male. Therefore, the abnormality arose in the female gamete during meiosis II.

b.

Turner syndrome is characterized by an absence of one of the sex chromosomes, and the individual has the genotype XO. Hemophilia is an X-linked recessive trait. The male parent is normal, he does not have hemophilia. The woman must be a carrier for the mutant hemophilia allele since her father is affected.

Therefore, the nondisjunction occurred in the father (male parent) during meiosis I. This cuases one of the gametes produced by the male parent with XY genotype and the other without the sex chromosome. If this gamete that lacks the sex chromosome fused with normal female gamete, the resulting child has the genotype XhO, and is affected by turner's syndrome. The X chromosome inherited from the female parent is mutated and has a defective ' h' allele for hemophilia, and thus, the child also got affected with hemophilia.

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