A form of dwarfism known as Ellis-van Creveld syndrome was first discovered in t
ID: 92925 • Letter: A
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
Answer:
Based on the information given:
This information indicates the most likely mode of inheritance is autosomal recessive.
In Autosomal recessive disorders affected person need to have two mutant alleles.
The parents of a person with an autosomal recessive disorder are usually not affected and are heterozygotes (e.g. Aa).
Therefore, the probability of having an affected child is 25% (if there are two carriers - that is both parents are carriers).
The probability that other children are a carrier for the disease is 75% (2/3). All the children will be affected with this disorder if both parent are also affected with this disorder.
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