What do calico cat fur, color blindness, and the bleeding disorder hemophilia have in common? These conditions are all results of a gene on the X chromosome. Illnesses caused by this type of genetic blip, passed on through one of the sex chromosomes, are known as sex-linked diseases.

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The majority of these diseases, such as Duchenne muscular dystrophy and fragile X syndrome, are the result of an X chromosome gene. As a result, they are much more common in men than in women. That’s because women are defined by an XX pair of sex chromosomes, while men have an XY set. In order for a female to have a sex-linked disease, she would have to inherit two affected X chromosomes—one from the mother and one from the father. If only one parent passes down a gene for the disease, the woman becomes a carrier of a recessive gene and can pass it on to her offspring.

Males, on the other hand, have only one X chromosome. If it has the abnormal gene, they get the disease. The Y does not contain nearly as many genes as the X chromosome, so it can’t override the recessive gene. However, the Y chromosome does have genetic details about the formation and function of the testes, so it’s necessary for development of a normal male body.

ADDITIONAL FACTS

  1. Queen Victoria of England (1837–1901) was a carrier of the sex-linked disease hemophilia, a condition in which the blood does not clot properly. Her great-grandson, Alexei (1904–1918), an heir to the Russian throne, developed the disease. His parents employed the mystic healer Rasputin (1872–1916) to help relieve Alexei’s pain and stem the bleeding; the power they yielded to the fame-hungry monk led to the Russian Revolution in 1917.
  2. Females who are carriers of a sex-linked disease have a normal X chromosome, which usually protects them so that they have no symptoms of the disease they carry. But in rare instances, it’s the normal X that is inactivated, and symptoms may occur.