Biology
Chapter 14
Sex-Linked Alleles
Sex-linked Genes
Special inheritance pattern for
genes on X chromosome
The Y
chromosome is
smaller than the X
and only has a few
genes
Sex-linked Genes
Special inheritance pattern for genes on X
chromosome
The Y chromosome is smaller than the X and
only has a few genes
Females get two copies of each gene on the X
chromosome
Males get only one copy of each gene on the X
chromosome
In males, any defect in their single X
chromosome is expressed.
Sex-linked Genes
Special inheritance pattern for genes on X chromosome
The Y chromosome is smaller than the X and only has a few genes
Females get two copies of each gene on the X chromosome
Males get only one copy of each gene on the X chromosome
In males, any defect in their single X chromosome is expressed.
Colorblindness (1 in 10 males, 1 in 100 females)
•Controlled by three genes on X chromosome
•In males, a defect in any one of them produces
red-green colorblindness
•Women are much less likely to have red-green
colorblindness because they have two copies of
each gene, two chances to get it right.
Males have only one X chromosome,
so all X-linked alleles are expressed in males
Colorblindness (1 in 10 males, 1 in 100 females) •Controlled by three genes on X
chromosome
•In males, a defect in any one of them produces red-green colorblindness
•Women are much less likely to have red-green colorblindness because they have two
copies of each gene, two chances to get it right.
Males have only one X chromosome,
so all X-linked alleles are expressed in males
Hemophilia (1 in 10,000 males, Extremely rare in females)
•Two genes that control blood clotting are
carried on 2 genes of X-chromosome
•A recessive allele at either gene may cause
hemophilia
•Resulting in a missing blood-clotting protein
Hemophilia (1 in 10,000 males, Extremely rare in females)
•Two genes that control blood clotting are carried on 2 genes
of X-chromosome
•A recessive allele at either gene may cause hemophilia
•Resulting in a missing blood-clotting protein
Duchenne Muscular Dystrophy
(1 in 3,000 males, Extremely rare in females)
•Progressive weakening of skeletal muscles
•Early death (rarely live past thirties)
X-linked Allele Puzzler
2. A man who suffers from hemophilia and a woman who does not
suffer from the condition have a daughter who is a hemophiliac. They
are about to have a second child, a son. What is the probability that
If wedraw a
their son will also be a hemophiliac? Explain your answer and
already
Punnett square to show your reasoning.
Father phenotype: hemophilia know this
next
Genotype: XhY
child is a
h
Y
X
boy,
it has a
Mother phenotype:
HXh
HY
H
X
X
X
50/50
Non-hemophilia
chance of
Genotype:
hY
?
h
h
h
X
XX
XX
H
?
having
X X
hemophilia.
Because this daughter is hemophiliac, mother must carry
The allele Xh.
X-linked Allele Puzzler
Hemophilia is a recessive allele on the Xchromosome, usually written Xh. Draw a Punnet
square to show the cross between a woman who
suffers from hemophilia and a man who does not.
Refer to your Punnett square to answer these
questions:
If they have a boy, what are the chances he will be a
hemophiliac?
If they have a girl, what is the probability she will
be a hemophiliac?
X-linked Allele Puzzler
Hemophilia is a recessive allele on the X-chromosome, usually written Xh. Draw a Punnet
square to show the cross between a woman who suffers from hemophilia and a man who
does not. Refer to your Punnett square to answer these questions:
If they have a boy, what are the chances he will be a hemophiliac?
If they have a girl, what is the probability she will be a hemophiliac?
Father phenotype: nonhemophilia
Genotype: XHY
Mother phenotype:
Hemophilia
Genotype:
XhXh
Xh
Xh
XH
Y
XHXh
XhY
XHXh
XhY
All of
thebeboys
will be hemophiliacs
None of the girls
will
hemophiliacs