Sex-Linked Inheritance
Gene Spy
Genetic counselors help prospective parents determine the probability of their children being
born with a genetic disorder. In this activity, you will act as a genetic counselor for a couple
with a family history of hemophilia who wish to start a family. Hemophilia, like other sexlinked traits, is determined by a gene on the sex chromosome and shows a different pattern of
inheritance in males than in females.
The possible sex-linked genotypes are as follows:
Unaffected men = XRY
Unaffected women = XR XR or XR Xr
Affected men = XrY
Affected women = Xr Xr
1. A genetic counselor begins with what is known about his or her current clients. In this
case, your male client HAS hemophilia.
This means that the man’s genotype is XrY
2. The female client does not display the symptoms of hemophilia. However, because she
was adopted, you know nothing of her genetic history and cannot be sure of her
genotype.
This means that the woman’s genotype is XR XR or XR Xr .
3. Using the space below, your next task is to use Punnett squares to determine all
possible genotype outcomes for this couple’s future children (the F1 generation). Since
you don’t know exactly what the mother’s genotype is, you will have to complete two
different Punnett square, one for each possibility.
Xr
Y
XRXr
XRY
XR
XR
XRXr
XRY
Xr
XR
Xr
Y
XRXr
XRY
XrXr
Xr Y
4. Use the results from question #3 to complete this table.
Mother’s Genotype
What percentage are
female AND affected?
What percentage are male
AND affected?
Homozygous Dominant
0%
0%
Heterozygous
25%
25%
Analyze and Conclude
5. After talking with the couple, they decide that the risk of their children having
hemophilia is not great enough to cause them to reconsider starting a family. Four
years later you find out that their first child – a girl – does not display any signs of
hemophilia. However, their second child – a boy – is born with the disorder. What does
this information now tell you about the mother’s genotype? Explain your answer.
The mother’s a carrier, so her genotype is heterozygous for the trait. This is because she
couldn’t have a child that had the disorder if she was homozygous dominant
6. What are the chances that the couple’s daughter is a carrier for the disease? How do
you know?
The chances that the couple’s daughter is a carrier for the disease are 25%. If the mother is
heterozygous as well as a carrier, the daughter has a 25% chance of carrying it.
7. If the daughter is a carrier for hemophilia and marries a non-hemophiliac man, could
any of her daughters have hemophilia? Her sons? Construct a Punnett square and
make a prediction on what you find out.
XR
XR
Y
Xr
XRXR
XRXr
XRY
XrY
8. In a population, 1 in 10,000 males will have hemophilia compared to only 1 in 1,000,000
females. Explain why this is so.
Males have the greater chance of getting hemophilia because they have one x
chromosome, and females have two which means males have a much greater chance.