JAMA. 1965;191(10):855. doi:10.1001/jama.1965.03080100073022.
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A heritable trait is said to have a sex-linked mode of transmission when the gene responsible for the trait is located on the X chromosome. According to classic genetics, the "carrier" female of a sex-linked trait should be normal and the male with the tainted X chromosome manifest the full syndrome. However, it is now known that carrier females frequently show some expressivity in sex-linked disorders, which may be explained by recent findings suggesting that the normal female of many species does not have two genetically active X chromosomes.1

Mammalian genetic studies indicate that one X chromosome in the female is destined to become inactive early in embryonic life, to form the deep-staining mass known as sex chromatin. The choice of which X chromosome will be inactivated in any particular cell is random; therefore, an X chromosome carrying a deleterious gene may be present throughout the organism, affecting some


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