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14–2 Human Chromosomes
14-2 Human Chromosomes
Photo credit: Richard Hutchings/Photo Researchers, Inc.
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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.
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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
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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.
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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.
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6. 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.”
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7. Chromosomal Disorders
If nondisjunction occurs, abnormal numbers of chromosomes may find their way into gametes, and a disorder of chromosome numbers may result.
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8. 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
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9. 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
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10. 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.
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11. 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.
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