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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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This note was uploaded on 11/03/2009 for the course BIOLOGY 1223 taught by Professor Gardiner/lajoie during the Fall '09 term at UWO.
- Fall '09