Week 2: Discussion Section Key
1a) What is the probability that a woman whose brother has Duchenne’s disease (an Xlinked rare trait) will have an
affected child?
(1/2)(1/4) = 1/8
b) The woman has another sister who has galactosemia, caused by an autosomal recessive allele. Her husband’s
father also had this disease. What is the chance their first child will have galactosemia?
(2/3)(1/4) = (1/6)
c) What is the probability that a man whose brother has Duchenne’s disease will have an affected child?
0
2) Consider a family where the mother’s sister has PKU and the father’s grandfather had PKU. This is known to be
caused by a rare autosomal recessive allele. They would like to have 5 children.
a) What is the chance that both parents are heterozygous for PKU?
(1/2)(2/3) = 1/3
b) What is the chance that their first child will have PKU?
(1/3)(1/4) = 1/12
c) If they have five children, what is the chance that two of the children will have PKU?
(2/3)(1/2)(5!/2!3!)(1/4)(1/4)(3/4)(3/4)(3/4) = 270/3072
e) If they have five children, what is the chance that at least two child will have PKU?
(1/3)[1 (3/4)^5] = 0.254 = 25.4%
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 Summer '08
 Ribaya
 PKU, autosomal recessive allele, aa black Aa, ac ac Agouti

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