A heritable trait is said to have a sex-linked mode of transmission when the gene responsible for the trait is located on one or both of the X chromosomes.* The female with her complement of two matched X chromosomes would not be expected to show the influence of a single sex-linked recessive gene when a normal gene is present on the fellow X chromosome. On the other hand, no such impunity exists for the male. Because the X and Y chromosomes in the male are not a matched pair, he expresses fully the effects of a single recessive gene. Thus sex-linked inheritance has as its hallmark the relatively normal carrier female and her affected male offspring. It has become increasingly apparent, however, that the heterozygous female, carrying one normal and one recessive gene on her sex chromosomes, may exhibit pathology similar to, but usually less severe than, that of the
GOODMAN G, RIPPS H, SIEGEL IM. Sex-Linked Ocular Disorders: Trait Expressivity in Males and Carrier Females. Arch Ophthalmol. 1965;73(3):387–398. doi:10.1001/archopht.1965.00970030389018
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