Download Human Genetics Practice Problems

Sex-linkage Practice Problems
Solve each of the following problems below. Show all of your work.
1. Hemophilia is a sex-linked recessive gene defect. Betsy and Bob are married and have two children named Billy and
Bell. Betsy is a carrier (heterozygous) for hemophilia and her husband, Bob, is normal (does not have hemophilia).
What is the probability that their daughter, Belle, will have hemophilia? What is the
probability that their son, Billy, will have hemophilia? Use XH, Xh, and Y for the
alleles.
a. What is Betsy’s genotype? ________________
b. What is Bob’s genotype? _________________
c. What are Billy’s possible genotypes? _______________ or ______________
d. What are Belle’s possible genotype? ________________ or ______________
2. Duchenne’s muscular dystrophy is caused by sex-linked recessive gene. Barbie is a carrier (heterozygous) for
Duchenne’s muscular dystrophy and her husband, Barney, is normal and does not have muscular dystrophy. What is
the probability that one of their children will be born with muscular dystrophy? Use XD for the dominant allele, Xd for the
recessive allele, and Y for the male chromosome.
a. What is Barbie’s genotype for hemophilia? ________________
b. What is Barney’s genotype (s) for hemophilia? _________________
c. What are the possible genotypes and phenotypes of their children?
____________________________________________________________________________________________
3. Colorblindness is caused by a sex-linked recessive allele. Betsy is a carrier (heterozygous) for colorblindness and Bob
is colorblind. Their oldest child, Billy is also colorblind, but Belle has normal vision. Use XC+, Xc, and Y for the alleles.
a. What is Betsy’s genotype for colorblindness? ________________
b. What is Bob’s genotype for colorblindness? _________________
c. What is Billy’s genotype for colorblindness? _________________
d. What is Belle’s genotype for colorblindness? ________________
e. Why is Billy colorblind but not Belle? ______________________________________________________________
____________________________________________________________________________________________
4. Barbie is a carrier (heterozygous) for the colorblindness trait and her husband, Barney, has normal vision. What are the
possible genotypes and phenotypes of their children?
a. What is Barbie’s genotype for colorblindness? ________________
b. What is Barney’s genotype for colorblindness? _________________
c. What are the possible genotypes and phenotypes of their children?
____________________________________________________________________________________________