Tay–Sachs disease is an autosomal recessive neurological disorder that is fatal in infancy. Despite its invariably lethal effect, Tay–Sachs disease occurs at very high frequency in some Central and Eastern European (Ashkenazi) Jewish populations. In certain Ashkenazi populations, 1 in 750 infants has Tay–Sachs disease. Population biologists believe the high frequency is a consequence of genetic bottlenecks caused by pogroms (genocide) that have reduced the population multiple times in the past several hundred years. Explain how a genetic bottleneck and its aftermath could result in a population that carries a lethal allele in high frequency.

Key Concepts

Genetic Bottleneck

A genetic bottleneck occurs when a population experiences a significant reduction in size due to environmental events or human activities, leading to a loss of genetic diversity. This reduction can result in certain alleles becoming more prevalent simply due to chance, as the surviving population may not represent the genetic diversity of the original group. Consequently, alleles that may be harmful, such as those causing Tay–Sachs disease, can become more common in the population.

Autosomal Recessive Inheritance

Autosomal recessive inheritance refers to a pattern where two copies of a mutated gene (one from each parent) are necessary for an individual to express a trait or disorder. In the case of Tay–Sachs disease, individuals must inherit two copies of the defective gene to develop the condition, while carriers with only one copy remain unaffected. This mode of inheritance can lead to a higher frequency of the allele in populations where carriers are more common due to historical factors like genetic bottlenecks.

Founder Effect

The founder effect is a phenomenon that occurs when a small group of individuals establishes a new population, leading to a limited genetic variation compared to the original population. If the founders carry certain alleles, including those for genetic disorders, these alleles can become disproportionately represented in the new population. In the context of Tay–Sachs disease, the historical isolation and demographic events affecting Ashkenazi Jews may have contributed to a higher frequency of the lethal allele due to the founder effect.