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SEX-LINKED TRAITS
Karyotype:
a picture of chromosomes.
…
Autosomes:
the first 22 homologous pairs of
chromosomes.
• Autosomes are the same for both
males and females.
Sex chromosomes:
determines the sex of the
individual.
• The sex chromosomes are the
23rd pair of chromosomes.
XX = female
XY = male
Which parent determines the sex of
an offspring?
DAD
Why?
• All moms have the genotype XX.
When egg cells are made, they
will all carry a single X
chromosome.
• All dads have the genotype XY.
When sperm cells are made, 50%
will have an X chromosome and
50% will have a Y chromosome.
• Therefore, males and females are
born in roughly a 50:50 ratio.
SEX-LINKED TRAITS:
those traits that are controlled by
genes on the X or Y
chromosomes.
• NOTE: The Y chromosome is much
smaller than the X chromosome and only
contains a few genes. Most sex-linked
traits are on the X chromosome.
In humans, hemophilia is a sex-linked trait.
Having hemophilia is recessive (Xh) to being
normal (XH). The heterozygous female is
called a carrier. Cross a carrier female with a
normal male.
XHXh X _____
In humans, hemophilia is a sex-linked trait.
Having hemophilia is recessive (Xh) to being
normal (XH). The heterozygous female is
called a carrier. Cross a carrier female with a
normal male.
XH Xh X XH Y
H
X
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X
Y
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X
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Y
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X X
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X X
Y
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X Y
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X
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X
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X X
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X X
Y
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X Y
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XY
Genotypic ratio:
1 XHXH :1XHXh :1XHY :1XhY
Phenotypic ratio:
2 normal females: 1normal male: 1
male with hemophilia
Cross a carrier female with a male
with hemophilia.
XHXh X _____
Cross a carrier female with a male
with hemophilia.
XHXh X XhY
H
X
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X
Y
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X
H
X
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X
Y
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X X
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XX
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X X
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XX
Y
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X Y
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X
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X
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X X
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XX
Y
H
X Y
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XY
Genotypic ratio:
1 XHXh :1XhXh :1XHY :1XhY
Phenotypic ratio:
1 normal female: 1 female with
hemophilia:1 normal male: 1 male
with hemophilia
In humans, red-green colorblindness is a sexlinked trait. People with red-green
colorblindness can not tell the difference
between red and green. Colorblindness is the
result of a recessive allele. Cross a female with
colorblindness with a male with normal vision.
XnXn X _____
In humans, red-green colorblindness is a sexlinked trait. People with red-green
colorblindness can not tell the difference
between red and green. Colorblindness is the
result of a recessive allele. Cross a female with
colorblindness with a male with normal vision.
X nX n X X N Y
n
X
N
X
Y
n
X
n
X
N
X
Y
N
n
X X
n
X
N
X
Y
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X X
N
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X X
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X
n
X
N
X
N
n
X X
N
n
X X
Y
n
XY
n
X
n
X
N
X
N
n
X X
N
n
X X
Y
n
XY
n
XY
Genotypic ratio:
2 XNXn : 2 XnY
Phenotypic ratio:
2 normal females: 2 males with
colorblindness
Why are sex-linked traits more
common in males than in females?
• Because a male only has to inherit
ONE recessive allele in order to get a
sex-linked trait and a female has to
inherit TWO recessive alleles in
order to acquire the sex-linked trait.
It is easier to inherit one recessive
allele than two.
If the female only inherits one
recessive allele, then they are a
carrier but have the normal
phenotype.