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Chapter 14: The Human Genome (14-2)
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Genes located on the sex chromosomes (X, Y) are
said to be “sex-linked”
More than 100 sex-linked genetic disorders have
now been mapped to the X-chromosome.
1 in 10 males in the U.S. and 1 in 100 females
 Males have just one X chromosome. Thus, all
X-linked alleles are expressed in males, even if
they are recessive.
 Females need two alleles for it to be expressed.
 Thus, much more common in males:
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Male
Female
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Normal
XCY
XCXC
Carrier
---XCXc
1 in 10,000 males
Cause: defect in clotting factor XIII
“Royal disease”
colorblind
XcY
XcXc
Chapter 14: The Human Genome (14-2)
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Progressive weakening and loss of skeletal muscle
Rarely live past early adulthood
1 in 3000 males
Cause: defect version of the gene that codes for a
muscle protein.
Mary Lyons—in female cells one X-chromosome
is randomly switched off … form a condensed
region in the nucleus called a Barr Body
Calico cats—mosaic pattern in furr, etc.
Nondisjunction- “not coming apart”—homologous
chromosomes fail to separate.
 If Nondisjunction occurs, abnormal number of
chromosomes may find their way into gametes, and
a disorder of chromosome numbers may result.
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Trisomy 21
1 in 800
Mild to severe mental retardation, birth defects,
health problems, etc.
Chapter 14: The Human Genome (14-2)
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Turner’s (genotype XO)
Klinfelter’s (XXY) … rarely (XXY or XXXXY)
No case of babies without an X
Y determines maleness.