Inheritance of Sex–Linked Genes

In addition to their role in determining sex, the sex chromosomes, especially X chromosomes, have genes for many characters unrelated to sex. A gene located on either sex chromosome is called a sex–linked gene , although in humans the term has historically referred specifically to a gene on the X chromosome. (Note the distinction between the terms sex–linked gene, referring to a gene on a sex chromosome, and linked genes, referring to genes on the same chromosome that tend to be inherited together.) Sex–linked genes in humans follow the same pattern of inheritance that Morgan observed for the eye–color locus in Drosophila. Fathers pass sex–linked alleles to all of their daughters but to none of their sons. In contrast, mothers can pass sex–linked alleles to both sons and daughters.

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The transmission of sex–linked recessive traits.  In this diagram, the superscript A represents a dominant allele carried on the X chromosome, and the superscript a represents a recessive allele. Imagine that this recessive allele is a mutation that causes a sex–linked disorder, such as color blindness. White boxes indicate unaffected individuals, light–coloured boxes indicate carriers, and dark–coloured boxes indicate individuals with the sex–linked disorder

 

If a sex–linked trait is due to a recessive allele, a female will express the phenotype only if she is a homozygote. Because males have only one locus, the terms homozygous and heterozygous lack meaning for describing their sex–linked genes (the term hemizygous is used in such cases). Any male receiving the recessive allele from his mother will express the trait. For this reason, far more males than females have sex–linked recessive disorders. However, even though the chance of a female inheriting a double dose of the mutant allele is much less than the probability of a male inheriting a single dose, there are females with sex–linked disorders. For instance, colour blindness is a mild disorder inherited as a sex–linked trait. A colour–blind daughter may be born to a colour–blind father whose mate is a carrier. However, because the sex–linked allele for colour blindness is relatively rare, the probability that such a man and woman will mate is low.

A number of human sex–linked disorders are much more serious than colour blindness. An example is Duchenne muscular dystrophy , which affects about one out of every 3,500 males born in the United States. The disease is characterised by a progressive weakening of the muscles and loss of coordination. Affected individuals rarely live past their early 20s. Researchers have traced the disorder to the absence of a key muscle protein called dystrophin and have mapped the gene for this protein to a specific locus on the X chromosome.

Haemophilia is a sex–linked recessive disorder defined by the absence of one or more of the proteins required for blood clotting. When a person with haemophilia is injured, bleeding is prolonged because a firm clot is slow to form. Small cuts in the skin are usually not a problem, but bleeding in the muscles or joints can be painful and can lead to serious damage. Today, people with haemophilia are treated as needed with intravenous injections of the missing protein.