Question

The frequency of a genetic syndrome is 1 in 2000. Assuming that the syndrome is determined by a homozygous recessive allele and that there are only 2 alleles in the population, what is the percentage of carriers for this syndrome assuming the gene is in HWE? What is the probability that a person, who discovers that they are a carrier, has a child with the syndrome?

Answer 1

There are 1 in 2000 individuals with recessive phenotype.

Frequency of recessive genotype= Number of individuals/Total number of individuals= 1/2000= 0.0005

q2= 0.0005

q= Frequency of recessive allele= square root of 0.0005= 0.02236= 0.022

As per Hardy Weinberg equation,

p+q= 1

p= frequency of dominant allele= 1-0.022= 0.978

Frequency of heterozygotes= 2pq= 2 X 0.978 X0.022= 0.043

Percentage of heterozygotes= 0.043X 100= 4.3%

Let the dominant genotype be H and recessive genotype be h. The heterozygotes will be Hh

		Hh
		H	h
Hh	H	Hh Homozygous dominant	Hh Heterozygotes
	h	Hh Heterozygotes	Hh Homozygous recessive

  The probability of obtaining a child with the syndrome (homozygous recessive) is 1/4 or 0.25.