Transcript Sex-Linked Genes

14–2 Human Chromosomes
Sex-Linked Genes
Sex-Linked Genes
The X chromosome and the Y chromosomes
determine sex.
Genes located on these chromosomes(X & Y) are
called sex-linked genes.
HOWEVER; MOST WE WILL DISCUSS ARE ON
THE X CHROMOSOME!
More than 100 sex-linked genetic disorders have
now been mapped to the X chromosome. Almost
all are recessive.
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14–2 Human Chromosomes
Sex-Linked Genes
X Chromosome
The Y chromosome is
much smaller than the X
chromosome and
appears to contain fewer
genes.
Duchenne muscular
dystrophy
Melanoma
X-inactivation center
X-linked severe combined
immunodeficiency (SCID)
Colorblindness
Hemophilia
Y Chromosome
Testis-determining
factor
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14–2 Human Chromosomes
Sex-Linked Genes
Why are sex-linked disorders more
common in males than in females?
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14–2 Human Chromosomes
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 Xlinked alleles are expressed in males, even if they
are recessive.
What does this mean…males only have to have
one recessive allele and they will have the
recessive phenotype.
Females with one allele for these conditions are
called carriers. They are “normal”.
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Ex:
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14–2 Human Chromosomes
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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14–2 Human Chromosomes
Sex-Linked Genes
Possible Inheritance of
Colorblindness Allele
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Hemophilia; “bleeder’s
disease”
14–2 Human Chromosomes
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Practice problem…
14–2 Human Chromosomes
IF an affected male (with hemophilia) marries a
woman who is a normal, what is the probability that
their children will have hemophilia?
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14–2 Human Chromosomes
X-Chromosome Inactivation
X-Chromosome Inactivation
British geneticist Mary Lyon discovered that in
female cells, 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.
This explains why XXX females don’t show
symptoms.
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14–2 Human Chromosomes
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14–2 Human Chromosomes
Tallulah
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14–2 Human Chromosomes
Ectodermal
dysplasias.
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14–2
Which of the following genotypes indicates an
individual who is a carrier for colorblindness?
a. XCX
b. XCXc
c. XcY
d. XCY
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14–2
Colorblindness is much more common in males
than in females because
a. the recessive gene on the male’s single X
chromosome is expressed.
b. genes on the Y chromosome make genes on
the X chromosome more active.
c. females cannot be colorblind.
d. 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
a. sex of an individual.
b. blood type of an individual.
c. chromosome number of an individual.
d. genotype of an individual.
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