1.
Duchenne muscular dystrophy is caused by a recessive allele located on the X
chromosome.
a. In the space provided, draw a Punnett square that shows how two unaffected people can have a child who has Duchenne muscular dystrophy.
Genotypic ratio:
Phenotypic ratio:
b. Is the child with muscular dystrophy male or female?
c. What is the probability that
the couple's next child will have muscular dystrophy? ______
d. What percentage of the couple's male children will
probably have muscular dystrophy? ______
e. What percentage of the couple's female children will
probably have muscular dystrophy? ______
2. Hemophilia is caused by a recessive allele on the X
chromosome.
a. In
the space provided, draw a Punnett square that shows how a mother whose blood
clots normally can have a daughter with hemophilia.
Genotypic ratio:
Phenotypic ratio:
b. What is
the father's genotype and phenotype?
c. Why is it
extremely uncommon for a female to have hemophilia?
3. Not all sex-linked genes are recessive. For example,
hypophosphatemia is caused by a dominant allele on the X chromosome.
a. Using A to represent the
allele for hypophosphatemia, draw a Punnett square that shows the possible offspring of a woman with genotype
XAXa and a man with genotype XaY
b. What percentage of the children have hypophosphatemia?
c. Are males more likely to have hypophosphatemia than
females? Explain.
d. Draw a Punnett square that shows the possible offspring
of a woman with a normal phenotype and a man with hypophosphatemia.
e. What percentage of the children have hypophosphatemia?
f. What percentage of the male
offspring have hypophosphatemia? Explain this result.
Genotypic ratio:
Phenotypic ratio: