A form of dwarfism known as Ellis–van Creveld syndrome was first discovered in the late 1930s, 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.

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?

Key Concepts

Genetic Drift

Genetic drift refers to the random fluctuations in allele frequencies within a population, particularly in small populations. In the case of the Pennsylvania Amish, the small size and isolation of their community may have led to a higher frequency of certain genetic traits, such as those causing Ellis–van Creveld syndrome, due to chance rather than natural selection.

Founder Effect

The founder effect occurs when a small group of individuals establishes a new population, carrying only a fraction of the genetic diversity of the original population. In this scenario, Samuel King and his wife, as founders of the Pennsylvania Amish community, may have carried alleles for Ellis–van Creveld syndrome, leading to a higher prevalence of the disorder among their descendants.

Autosomal Recessive Inheritance

Ellis–van Creveld syndrome is inherited in an autosomal recessive manner, meaning that an individual must inherit two copies of the mutated gene (one from each parent) to express the disorder. Since all affected individuals in the Pennsylvania Amish community have unaffected parents, it suggests that both parents are carriers of the recessive allele, which can lead to a higher incidence of the disorder in a population with limited genetic diversity.