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 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 Amish population in eastern Pennsylvania by Victor
McKusick and his colleagues (1964). In that community, about 5 per 1000 births
are affected, and in the population of \(8000,\) 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 was affected with
the 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 HardyWeinberg law, calculate the frequency of the
mutant allele in the population and the frequency of 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?
