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

Question

A form of dwarfism known as Ellis-van Creveld syndrome was first discovered in the late 1930's, when Richard Ellis and Simon van Creveld shared a train compartment on their way to a pediatrics conference, and discovered 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 syndrome was reported in an Old Order Amish population in eastern Pennsylvania in 1964. In this community with a population of 8000, the observed frequency is 2 per 1000. All affected individuals have unaffected parents, and all affected cases can be traced back to Samuel King and his wife, who arrived in the area in 1774. Neither King nor his wife was affected with the disorder. There are no known cases of the disorder in other Amish communities, such as those in Ohio and Indiana.

From the information given, hypothesize the most likely mode of inheritance (Dominant or recessive? Autosomal or X-linked?) for this disorder. Determine the frequency of the wild-type and Ellis-van Creveld alleles, and calculate the genotype frequencies in the population. What is the most likely explanation for the high frequency of the disorder in the Pennsylvania Amish communities?

Explanation / Answer

From the provided information, it can be stated that the mode of inheritance of Ellis-van Creveld syndrome is Autosomal Recessive Mode as

Also, it is known that ACHONDROPLASTA (Dwarfism: Defects in long bone growth) and POLYDACTYLY (More than five fingers in hands or toes) individually are considered to have AUTOSOMAL DOMINANT MODE OF INHERITANCE but in the given case these Syndromes are provided as symptoms. Thus, It can be concluded that the mode of inheritance of Ellis-van Creveld syndrome is Autosomal Recessive Mode.

CALCULATING ALLELE AND GENOTYPE FREQUENCY:

GIVEN: In this community with a population of 8000, the observed frequency is 2 per 1000.

q2 = 2 x 10-3 = 0.002

Frequency of Ellis-van Creveld allele, q = 0.045

We know that, p + q = 1

So, p = 1 - q

Frequency of wild type allele, p = 1 - 0.045 = 0.955 frequency of wild type allele

Genotype frequency:

AA = p2 = 0.955 x 0.955 = 0.912025

Aa = 2pq = 2 x 0.955 x 0.045 = 0.08595

aa = q2 = 0.002

The most likely explanation for the high frequency of the disorder in the Pennsylvania Amish communities can be described as Initial founder Effect as the King and Queen were possessing lethal alleles which later spread within the population because of inbreeding.

Thank you.

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