Introduction to Genetic Analysis 55

Introduction to Genetic Analysis 55 - 44200_02_p27-72...

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women whose sisters’ sons had bled profusely when they were circumcised. Duchenne muscular dystrophy is a fatal X-linked re- cessive disease. The phenotype is a wasting and atrophy of muscles. Generally the onset is before the age of 6, with confinement to a wheelchair by 12, and death by 20. The gene for Duchenne muscular dystrophy has now been isolated and shown to encode the muscle protein dystrophin. This discovery holds out hope for a better understanding of the physiology of this condition and, ultimately, a therapy. A rare X-linked recessive phenotype that is interest- ing from the point of view of sexual differentiation is a condition called testicular feminization syndrome, which has a frequency of about 1 in 65,000 male births. People afflicted with this syndrome are chromosomally males, having 44 autosomes plus an X and a Y, but they de- velop as females (Figure 2-27). They have female exter- nal genitalia, a blind vagina, and no uterus. Testes may be present either in the labia or in the abdomen. Although
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