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.

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1. A genetic bottleneck occurs when a population undergoes a drastic reduction in size due to events such as natural disasters, disease outbreaks, or human actions like genocide. This reduction limits genetic diversity because only a small subset of individuals survive and reproduce.
2. During a bottleneck, alleles present in the surviving population may become disproportionately represented, regardless of whether they are beneficial, neutral, or harmful. This is due to random chance rather than natural selection.
3. If the surviving population includes individuals carrying a lethal recessive allele (such as the Tay–Sachs allele), the frequency of this allele can increase in the population simply because it is present in the reduced gene pool.
4. After the bottleneck, the population may begin to grow again, but the genetic diversity remains limited. The previously rare lethal allele may persist at a higher frequency because there is less genetic variation to dilute its presence.
5. In subsequent generations, the allele frequency can remain high, especially if the population remains relatively isolated (e.g., through cultural or geographic factors) and interbreeding within the group continues. This phenomenon is known as genetic drift, which can amplify the effects of the bottleneck over time.

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 in the surviving population, as the genetic variation is limited to the alleles present in the few individuals that survive. Consequently, harmful alleles, such as those causing Tay-Sachs disease, can persist at higher frequencies.

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 the defective allele from both parents to develop the condition. This means that carriers, who possess only one copy of the allele, do not show symptoms but can pass the allele to their offspring, allowing the allele to remain in the population.

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 pool. If the founders carry certain alleles, including those for genetic disorders, these alleles can become more common in the new population due to the reduced genetic diversity. In the context of Ashkenazi Jews, historical events that led to population bottlenecks may have resulted in a higher frequency of the Tay-Sachs allele among descendants.