SBI 3U
Unit 2: Genetics
Problem Set 5: Sex-Linked Crosses
1. Diagram a cross between a male and a female, showing only X and Y chromosomes. What proportion
of males and females is expected from such a cross?
2. Red-green color blindness is caused by a sex-linked recessive gene. Using C for normal vision and c
for colorblindness, XX for female and XY for male, the following representations can be made:
XCXC
normal female
XC Xc carrier female
Xc Xc
colorblind female
XC Y
normal male
Xc Y
colorblind male
Determine the following crosses and give the expected phenotype and genotype ratios in the offspring.
Remember the sex of the individual as well as his or her genetic status must be given.
a. Colorblind male X
normal female
b. Colorblind male X
carrier female
c. Normal male
X
carrier female
3. Hemophilia, a genetic disorder in blood clotting, is caused by a sex-linked recessive gene.
a. Using H to represent the gene for normal clotting and h to represent the gene for hemophilia,
how would you represent each of the following:
i. normal male
ii. normal female
iii. hemophiliac male
iv. hemophiliac female
v. carrier female
b. Diagram each of the following crosses, giving the expected phenotype and genotype ratios in
the offspring.
i. Carrier female X
normal male
ii. Normal female X
hemophiliac male
iii. Carrier female X
hemophiliac male
4. A sex-linked gene c produces colorblindness in humans. A normal woman whose father was colorblind
marries a colorblind man.
a. What is the genotype of the woman
b. What are the possible genotypes of the mother of the colorblind man?
c. Suggest the possible genotypes of the children of this couple.
5. The alleles for eye color and for body colour are on the X chromosome of Drosophila, but not on the Y.
Red eye colour is dominant to white eye colour, and tan body colour is dominant to yellow body colour.
a. What is the genotype of a yellow-bodied, red-eyed female who is homozygous for eye color?
b. What is the genotype of a tan-bodied, white-eyed male?
c. What offspring would you expect from a cross between the female in part a, and the male in part
b?