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Biology 22
Problems in Mendelian Genetics
7. Tay-Sachs disease is a serious neurodegenerative disorder occurring in
individuals homozygous for a recessive allele, t.
a. Mr. and Mrs. D are both unaffected by Tay-Sachs disease and have
both a daughter with symptoms of this disease and a normal son (Son
D). What is the probability that Son D is a carrier of the recessive
allele?
b. Ms. E is unaffected by Tay-Sachs disease but has a brother with the
disease. If Son D marries Ms. E, what is the probability that their first
offspring will be affected by Tay-Sachs disease?
c. If Son D and Ms. E have four children, what is the probability that two
would be affected by Tay-Sachs disease and two would be
unaffected?
d. If the first offspring of Son D and Ms. E is an unaffected female, what is
the probability that the second child of this marriage would be an
unaffected male?
e. If the first child born to Son D and Ms. E is affected by Tay-Sachs
disease, what is the probability that their second child would be
affected by Tay-Sachs disease?
f. If the first child born to Son D and Ms. E is an affected female, what is
the probability that the second child will be an affected male and that
the third child will be an unaffected male?
8. Hemophilia is an X-linked recessive trait in humans. Huntington’s Disease is
inherited with an autosomal dominant allele.
a. Mr. Y is unaffected by either condition. He marries Ms. X, who is
unaffected by hemophilia but shows signs of Huntington’s Disease.
Ms. X’s father has hemophilia but is unaffected by Huntington’s
Disease. What is the probability that the first child born to Ms. X and
Mr. Y will be a son with hemophilia and their second child will be a
daughter with Huntington’s Disease and their third child will be a son
who is unaffected by either condition?
b. For Ms. X and Mr. Y, what is the probability that the first child will be
either a son with Huntington’s Disease but without hemophilia or a
daughter that is unaffected by Huntington’s Disease but carries the
allele for hemophilia?
c. Mr. U has hemophilia and Huntington’s Disease. Mr. U’s father is
unaffected by either condition. Mr. U marries Ms. V, who is unaffected
by either condition. Ms. V’s father has hemophilia. Out of three
children, what is the probability that two will be girls with hemophilia
and Huntington’s Disease?
d. What is the probability that Mr. U and Ms. V will have a son with
hemophilia or a daughter with Huntington’s Disease as their first child?
e. What is the probability that Mr. U and Ms. V will have a son with
hemophilia and Huntington’s Disease or a daughter with neither
condition as their second child?