Phenylketonuria (PKU) is a disease caused by the build-up of the byproducts of metabolizing phenylalanine. It is caused by a defective, recessive allele. If a child is homozygous for this recessive allele, it will develop PKU. In the United States, PKU is detected in approximately 1 in 10,000 live births. What is the frequency of the recessive allele in the population? What proportion of people in our country are expected to be carriers (i.e. heterozygotes)?
Example:
I go to a different population of fruit flies that have the same two alleles for eye-color. I suspect that
the alleles occur in different frequencies in this second population. I sample 1000 flies and discover
10 that have brown eyes. What are the estimated frequencies of the "R" and "r" alleles in this
population?
Answer: Again, p2 + 2pq + q2 = 1. The term q2 = the relative frequency of homozygous recessive
individuals, which corresponds to the ten brown-eyed flies I counted out of 1000 flies sampled. Thus,
q2 = 10/1000 = 1/100. q = the square root of 1/100 or 0.1. Thus the frequency of "r" in this second
population is 0.1 and the frequency of the "R" allele is 1 - q or 0.9.
Problem 1:
Phenylketonuria (PKU) is a disease caused by the build-up of the byproducts of
phenylalanine. It is caused by a defective, recessive allele. If a child is homozygous for this recessive
allele, it will develop PKU. In the United States, PKU is detected in approximately 1 in 10,000 live
births. What is the frequency of the recessive allele in the population? What proportion of people in
our country are expected to be carriers (i.e. heterozygotes)?
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