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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
Why are sex-linked disorders more common in males than in females?
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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.
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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.
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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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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.
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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.
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11. 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.
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14–2
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14–2
The average human gene consists of how many base pairs of DNA?
3000
300 20
30,000
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14–2
Which of the following genotypes indicates an individual who is a carrier for colorblindness?
XCX
XCXc
XcY
XCY
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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.
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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.
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14–2
Nondisjunction occurs during
meiosis I.
mitosis.
meiosis II.
between meiosis I and II.
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18. END OF SECTION