Dear List members
I have been involved in setting assessment questions about family and
population genetics for our MBBS course. I have been doing this with a
geneticist. However, we disagree with regards one of the solutions. I
would welcome your opinions. I appreciate that this is a "specialist"
topic - I hope that it is not to obscure.
Below is the question which consists of three parts. I have provided
the answers to parts a) and b) - not least to help those intersted in
helping me but perhaps not too familiar with the topic. It is the answer
to part c) that I am most interested in. At the moment it is best if I
do not indicate where the geneticist and I disagree.
By the way, this is not my homework.
Many thanks
Philip
Question
Phenylketonuria is an autosomal recessive disease in which the
metabolism of phenylalanine is blocked. This leads to a phenotype which
may include mental retardation, pigmentation changes, eczema and
epilepsy. Phenylketonuria has a frequency of 1/10000 affected newborns
in the Northern European population. The Hardy-Weinberg law can be used
to approximate the carrier frequency in this population.
(a) What is the Hardy-Weinberg law?
ANSWER:The Hardy-Weinberg law explains the relationship between allele
frequencies and genotypes in a population. Consider two alleles at a
locus, A and a, with frequencies of the allele A in the population given
by p and that for a being q. Obviously we must have (p+q) = 1.
Therefore, the proportion of the genotype AA in the population will be
p2, for Aa it will 2pq and that for aa is q2. Note that:
(b) Estimate the carrier frequency for phenylketonuria.
ANSWER:Phenylketonuria is autosomnal recessive with 1/10000 newborns
affected. Therefore frequency (aa) = 1/10000 = q2. Hence q=1/100.
As p+q = 1 then p =99/100.
Carriers are people with genotype Aa, frequency given by 2pq.
Therefore, the population frequency of carriers is given by
2 x 1/100 x 99/100 = 198/10000
(c) If the parent of a child with phenylketonuria remarries and the new
spouse is unrelated and unaffected, what is the risk of them having an
affected child?
_________________________________________________________
Dr. Philip Sedgwick
Lecturer in Medical Statistics
Department of Public Health Sciences
St. Georges Hospital Medical School
London SW17 0RE
Email: [log in to unmask]
Telephone: +44 20 8725 5551
Fax: +44 20 8725 3584
Webpage: http://www.sghms.ac.uk/depts/phs/staff/ps/ps.htm
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