Chapter 17. Inheritance of Sex Chromosomes, Linked Genes, and Organelles

Genes in the X chromosome show characteristic patterns in human pedigrees.

The features of X-linked inheritance can be seen in human pedigrees for traits due to an X-linked recessive mutation. These are illustrated in Fig. 17.7 for red–green color blindness, a condition that affects about 1 in 20 males. An individual with red–green color blindness will have difficulty seeing the number in the colored dots in Fig. 17.7.

FIG. 17.7 Inheritance of an X-linked recessive mutation. This pedigree shows the inheritance of red–green color blindness, an X-linked recessive mutation.

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The key features of the inheritance of traits due to rare X-linked recessive alleles, which are noted in the pedigree, are listed here:

Affected individuals are usually males because males need only one copy of the mutant gene to be affected, whereas females need two copies to be affected.
Affected males have unaffected sons because males transmit their X chromosome only to their daughters.
A female whose father is affected can have affected sons because such a female must be a heterozygous carrier of the recessive mutant allele.

An additional feature worth mentioning is that the sisters of an affected male each have a 50% chance of being a heterozygous carrier because when a brother is affected the mother must be heterozygous for the recessive allele.

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Quick Check 1 Is it possible for an unaffected female to have female offspring with red–green color blindness?

Quick Check 1 Answer

A woman whose father is color blind must be heterozygous for the mutant allele. If she has children with a man who is color blind, then half of the female offspring are expected to be homozygous mutant and therefore color blind.

A pedigree for one of the most famous examples of human X-linked inheritance is shown in Fig. 17.8. The trait is a form of hemophilia, which results from a recessive mutation in a gene encoding a protein necessary for blood clotting. Affected individuals bleed excessively from even minor cuts and bruises, and internal bleeding can cause excruciating pain. Affecting about 1 in 7000 males, hemophilia is famous because of its presence in many members of European royalty descended from Queen Victoria of England (1819–1901), who was a heterozygous carrier of the gene. By the marriages of her carrier granddaughters, the gene was introduced into the royal houses of Germany, Russia, and Spain. The mutant allele is not present in the present royal family of Britain, however, because this family descends from King Edward VII, one of Victoria’s four sons, who was not himself affected and therefore passed only a normal X chromosome to his descendants.

FIG. 17.8 X-linked hemophilia in European royalty.

The source of Queen Victoria’s hemophilia mutation is not known. None of her ancestors is reported as having a bleeding disorder. Quite possibly the mutation was present for a few generations before Victoria was born but remained hidden because it was passed from heterozygous female to heterozygous female.