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Practice Probability Problems
A. Probability in pedigrees:
1. The trait in the pedigree at the right is inherited from a single dominant gene.
i) What is the probability that II-1 is heterozygous? ii) What is the probability of an
affected child if the following cousins marry? a) III-1 X III-3; b) III-2 X III-4.
2. The trait in the pedigree below is inherited as a simple recessive. It is very rare in the general population, so assume
that anyone who marries into the family is homozygous for the normal allele unless there is evidence to the contrary.
What is the probability of the trait appearing in the offspring if the following individuals should marry?
a) III-1 XIII-12; b) III-4 X III-15; c) III-6 X III-13; d) IV-1 X IV- 2. (Tip! What is the probability that a normal offspring
of two carriers is heterozygous?)
B. Probability in families
1. What is the probability in families with six children will have a) one boy and five girls; b) three boys and three girls;
and c) all six the same sex?
2. Phenylketonuria in humans is inherited as a recessive condition. If both parents are heterozygous, what is the chance
that their five children will have the following phenotypes: a) all normal; b) four normal and one affected; c) three normal
and two affected; d) two normal and three affected; e) one normal and four affected; and f) all five are affected?
3.. Four roan cows are bred by the same roan bull. What is the probability that the calves will include one red, two roan,
and one white?
Answers
A-1-i) 1; ii-a) 0; 11-b) 1/2 A-2-a) 1/8; b) 1/16; c) 1/6; d) 1/24
B-1 a)6/64; b) 20/64; c) 1/64 + 1/64 =1/32 B-2 a) 245/1024; b) 405/1024;
c)270/1024; d) 90/1024; e) 15/1024 f) 1/1024 B-3) 3/16
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