A form of dwarfism know as Ellis-van Creveld. syndrome was first discovered in t

Question

A form of dwarfism know as Ellis-van Creveld. syndrome was first discovered in the late 1930s, when Richard Ellis and Simon van GigyglCl shared a train compartment on the way to a pediatrics meeting. In the course of conversation, they discovered that they each had a patient with this syndrome. They published a description of the syndrome in 1940. Affected individuals have a short-limbed form of dwarfism and often have defects of the lips and teeth, and Polydactyly (extra fingers). The largest pedigree for the condition was reported in an old order A mish population in eastern Pennsylvania by Victor McKusick and his colleagues (1964). In that community, about 5 per 1000 births are affected, and in the larger population of 8000 containing that community, the observed frequency is 2 per 1000. All affected individuals have unaffected parents, and all affected cases can trace their ancestry to Samuel King and his wife, who arrived in the area in 1774. It is known that neither King nor his wife were affected with disorder. There are no cases of the disorder in other Amish communities, such as those in Ohio or Indiana. a. From the information provided, derive the most likely mode of inheritance of this disorder. Using the Hardy-Weinberg equations, calculate the frequency of the mutant allele in the larger population and the frequency of the heterozygotes, assuming Hardy-Weinberg conditions. b. What is the most likely explanation for the high frequency of the disorder in the Pennsylvania Amish community and its absence in other Amish communities?

Explanation / Answer

The gene is recessive because the parents are not affected and they both have to be carriers to have a child with the disorder.

Let frequency of recessive allele in the population to be represented by q.

As the population is in Hardy-Wienberg equilibrium, then q^2 is the frequency of the homozygous recessive genotype in the population

q^2 = 5/1000

q^2 = 0.005

q = 0.071

Similarly, for larger population

q^2 = 2/1000

q^2 = 0.002

q = 0.045

p+q =1

So, p = 1 – 0.045 = 0.955

Frquency of heterozygous in population

2pq = 2* 0.955 * 0.045 = 0.086

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