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14–2 Human Chromosomes 14-2 Human Chromosomes Photo credit: Richard Hutchings/Photo Researchers, Inc. Copyright Pearson Prentice Hall
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Sex-Linked Genes Sex-Linked Genes The X chromosome and the Y chromosomes determine sex. Genes located on these chromosomes are called sex-linked genes. More than 100 sex-linked genetic disorders have now been mapped to the X chromosome. Copyright Pearson Prentice Hall
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Sex-Linked Genes X Chromosome Duchenne muscular dystrophy The Y chromosome is much smaller than the X chromosome and appears to contain only a few genes. Melanoma X-inactivation center X-linked severe combined immunodeficiency (SCID) Colorblindness Hemophilia Genes on X and Y chromosomes, such as those shown in the diagrams, are called sex-linked genes. Y Chromosome Testis-determining factor Copyright Pearson Prentice Hall
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Sex-Linked Genes Why are sex-linked disorders more common in males than in females? Copyright Pearson Prentice Hall
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Sex-linked genes In females, traits are only evident when both alleles are recessive. Females can be carriers. In males, having the trait is more common than in females because there is no 2nd X chromosome to mask the one X chromosome present. Copyright Pearson Prentice Hall
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Sex-Linked Genes Colorblindness Three human genes associated with color vision are located on the X chromosome. In males, a defective version of any one of these genes produces colorblindness. Copyright Pearson Prentice Hall
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Sex-Linked Genes Possible Inheritance of Colorblindness Allele X-linked alleles are always expressed in males, because males have only one X chromosome. Males who receive the recessive Xc allele all have colorblindness. Females, however, will have colorblindness only if they receive two Xc alleles. Copyright Pearson Prentice Hall
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Sex-Linked Genes Hemophilia The X chromosome also carries genes that help control blood clotting. A recessive allele in either of these two genes may produce hemophilia. In hemophilia, a protein necessary for normal blood clotting is missing. Hemophiliacs can bleed to death from cuts and may suffer internal bleeding if bruised. Copyright Pearson Prentice Hall
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Sex-Linked Genes Duchenne Muscular Dystrophy Duchenne muscular dystrophy is a sex-linked disorder that results in the weakening and loss of skeletal muscle. It is caused by a defective version of the gene that codes for a muscle protein. Copyright Pearson Prentice Hall
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X-Chromosome Inactivation
British geneticist Mary Lyon discovered that in female cells, genes of one X chromosome is randomly switched off. This chromosome forms a dense region in the nucleus known as a Barr body. Barr bodies are generally not found in males because their single X chromosome is still active. Copyright Pearson Prentice Hall
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14–2 Copyright Pearson Prentice Hall
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14–2 The average human gene consists of how many base pairs of DNA? 3000 300 20 30,000 Copyright Pearson Prentice Hall
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14–2 Which of the following genotypes indicates an individual who is a carrier for colorblindness? XCX XCXc XcY XCY Copyright Pearson Prentice Hall
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14–2 Colorblindness is much more common in males than in females because the recessive gene on the male’s single X chromosome is expressed. genes on the Y chromosome make genes on the X chromosome more active. females cannot be colorblind. colorblindness is dominant in males and recessive in females. Copyright Pearson Prentice Hall
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14–2 The presence of a dense region in the nucleus of a cell can be used to determine the sex of an individual. blood type of an individual. chromosome number of an individual. genotype of an individual. Copyright Pearson Prentice Hall
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14–2 Nondisjunction occurs during meiosis I. mitosis. meiosis II. between meiosis I and II. Copyright Pearson Prentice Hall
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