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Copyright Pearson Prentice Hall
14–2 Human Chromosomes 14-2 Human Chromosomes Photo credit: Richard Hutchings/Photo Researchers, Inc. Copyright Pearson Prentice Hall
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Copyright Pearson Prentice Hall
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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Copyright Pearson Prentice Hall
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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Copyright Pearson Prentice Hall
Sex-Linked Genes For a recessive allele to be expressed in females, there must be two copies of the allele, one on each of the two X chromosomes. Males have just one X chromosome. Thus, all X-linked alleles are expressed in males, even if they are recessive. Copyright Pearson Prentice Hall
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Copyright Pearson Prentice Hall
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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Chromosomal Disorders
The most common error in meiosis occurs when homologous chromosomes fail to separate. This is known as nondisjunction, which means, “not coming apart.” Copyright Pearson Prentice Hall
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Chromosomal Disorders
If nondisjunction occurs, abnormal numbers of chromosomes may find their way into gametes, and a disorder of chromosome numbers may result. Copyright Pearson Prentice Hall
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Chromosomal Disorders
Nondisjunction Homologous chromosomes fail to separate. Meiosis I: Nondisjunction Nondisjunction causes gametes to have abnormal numbers of chromosomes. The result of nondisjunction may be a chromosome disorder such as Down syndrome. Meiosis II Copyright Pearson Prentice Hall
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Chromosomal Disorders
Down Syndrome Karyotype Down syndrome produces mild to severe mental retardation. It is characterized by: increased susceptibility to many diseases higher frequency of some birth defects This karyotype is from a person with Down syndrome. Down syndrome causes mental retardation and various physical problems. People with Down syndrome can, however, lead active, happy lives. Photo credit: ©Dr. Dennis Kunkel/CNRI/Phototake Copyright Pearson Prentice Hall
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Chromosomal Disorders
Sex Chromosome Disorders In females, nondisjunction can lead to Turner’s syndrome. A female with Turner’s syndrome usually inherits only one X chromosome (karyotype 45,X). Women with Turner’s syndrome are sterile. Copyright Pearson Prentice Hall
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Chromosomal Disorders
In males, nondisjunction causes Klinefelter’s syndrome (karyotype 47,XXY). The extra X chromosome interferes with meiosis and usually prevents these individuals from reproducing. Copyright Pearson Prentice Hall
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