Study Guide 5 Sex-Linked Inheritance (Hemophilia)

Question 1

In humans, a single gene found on the X chromosome controls for the disease hemophilia. Individuals with hemophilia lack a protein required for clotting blood. When injured, hemophiliacs bleed excessively, and may even bleed to death after relatively minor cuts or bruises. The hemophilia gene has two alleles, H and h. The trait exhibits simple dominance, with the dominant phenotype of normal protein production (XH) and the recessive phenotype of hemophilia (Xh). In the table below, indicate
the sex of each patient, and whether he or she is healthy, a carrier (carries the hemophiliac gene, but does not suffer from the disease), or a hemophiliac.

Answer 1

Question 2

Show the cross of a man who has hemophilia and a woman who is a carrier. What is the probability that their children will have the disease? How does that probability change with the sex of the child?

Answer 2

Overall, 50% of offspring will have hemophilia. In this cross, the
probability is the same in both sexes

Question 3

A woman who is a carrier of hemophilia has children with a non-hemophilliac man. Show the cross and determine the probability that their children will have hemophilia. What sex will any hemophiliac children be?

Answer 3

25% of children will have hemophilia, all will be male
Daughters will be 50% healthy and 50% carriers

Question 4

A woman who has hemophilia has children with a non-hemophiliac man. Show the cross and determine how many of their children you would expect to have hemophilia and what the sex of the hemophiliac children will be.

Answer 4

All male offspring will have hemophilia (expected 50% of total
offspring). All female offspring will be carriers.

Question 5

Why are X-linked recessive diseases more common in men than women?

Answer 5

Since men only have one X chromosome, a single disease allele will cause them to exhibit the
disease. With two X chromosomes, women must inherit two disease alleles in order to exhibit the
disease. Since these alleles are often somewhat rare, it is less likely for a woman to inherit two
disease alleles than for a man to inherit one.

Question 6

If a man has an X-linked disease, what proportion of his sons would you expect to inherit that disease from him? Explain your answer

Answer 6

None. A man inherits a Y chromosome from his father. If he has the disease, he inherited it from his
mother through the X chromosome he inherited from her.