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Unformatted text preview: In addition to their role in determining sex, the sex chromosomes, especially the X chromosome, have genes for many characters unrelated to sex. These sex-linked genes follow the same pattern of inheritance as the white-eye locus in Drosophila . Sex-linked genes have unique patterns of inheritance If a sex-linked trait is due to a recessive allele, a female has this phenotype only if homozygous. Heterozygous females will be carriers. Because males have only one X chromosome ( hemizygous ), any male receiving the recessive allele from his mother will express the trait. The chance of a female inheriting a double dose of the mutant allele is much less than the chance of a male inheriting a single dose. Therefore, males are far more likely to inherit sex-linked recessive disorders than are females. Several serious human disorders are sex- linked. Duchenne muscular dystrophy affects one in 3,500 males born in the United States. Affected individuals rarely live past their early 20s. This disorder is due to the absence of an X- linked gene for a key muscle protein, called dystrophin . The disease is characterized by a progressive weakening of the muscles and loss of coordination. Hemophilia is a sex-linked recessive trait defined by the absence of one or more clotting factors. These proteins normally slow and then stop bleeding. Individuals with hemophilia have prolonged bleeding because a firm clot forms slowly. Bleeding in muscles and joints can be painful and lead to serious damage. Individuals can be treated with intravenous injections of the missing protein. Fig. 12.9 Although female mammals inherit two X chromosomes, only one X chromosome is active. Therefore, males and females have the same effective dose (one copy ) of genes on the X chromosome. During female development, one X chromosome per cell condenses into a compact object, a Barr body . This inactivates most of its genes. The condensed Barr body chromosome is reactivated in ovarian cells that produce ova. Male no Barr Body Females Barr Body Mary Lyon, a British geneticist, has demonstrated that the selection of which X chromosome to form the Barr body occurs randomly and independently in embryonic cells at the time of X inactivation. As a consequence, females consist of a mosaic of cells, some with an active paternal X, others with an active maternal X. After Barr body formation, all descendent cells have the same inactive X. If a female is heterozygous for a sex-linked trait, approximately half her cells will express one allele and the other half will express the other allele....
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- Fall '09