1. Chapter 14 Sex-Linked Alleles
Biology
Chapter 14
Sex-Linked Alleles

2. 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

3. 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.

4. Colorblindness (1 in 10 males, 1 in 100 females)
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

5. 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

6. 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

7. Duchenne Muscular Dystrophy •Progressive weakening of skeletal muscles
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)

8. 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 their son will also be a hemophiliac? Explain your answer and draw a Punnett square to show your reasoning.
If we already
know this next
child is a boy,
it has a 50/50
chance of having
hemophilia.
Father phenotype: hemophilia
Genotype: XhY Y Xh
Mother phenotype:
Non-hemophilia
Genotype:
XHX?
XHXh
XHY XH
XhY
Because this daughter is hemophiliac, mother must carry
The allele Xh. Xh X?
XhXh

9. 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?

10. 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 Y XH
Mother phenotype:
Hemophilia
Genotype:
XhXh
All of the boys will be hemophiliacs
None of the girls will be hemophiliacs
XHXh
XhY Xh
XhY Xh
XHXh