Question

A number of comparisons of nucleotide sequences among hominids and rodents indicate that inbreeding may have occurred more often in hominid than in rodent ancestry. Bakewell et al. \((2007 . \text { Proc. Nat. Acad. Sci }[\mathrm{USA}] 104: 7489-7494)\) suggest that an ancient population bottleneck that left approximately 10,000 humans might have caused early humans to have a greater chance of genetic disease. Why would a population bottleneck influence the frequency of genetic disease?

Short answer

Answer: A population bottleneck influences the frequency of genetic diseases by reducing genetic diversity, increasing the chances of inbreeding, and resulting in higher percentages of homozygosity within the population. This leads to an increased likelihood of offspring carrying harmful alleles and developing genetic diseases.

Step-by-step solution

Definition of Population Bottleneck

A population bottleneck occurs when a significant reduction in population size takes place due to a catastrophic event like natural disasters, human intervention, or environmental changes. This leads to reduced genetic diversity and a lack of certain alleles.

Reduction in Genetic Diversity

Due to the reduced population size, the remaining individuals of the species are less likely to possess the full range of genetic variation previously found within the population. This results in a decrease in genetic diversity and may lead to the loss of certain beneficial alleles.

Inbreeding and Increased Frequency of Genetic Diseases

When a population bottleneck occurs, the chances of mating among close relatives or inbreeding increase due to the lack of unrelated mates. When closely related individuals reproduce, they are more likely to share identical alleles. This results in offspring with an increased chance of inheriting harmful, recessive alleles from both parents, leading to a higher frequency of genetic diseases in the population.

Loss of Heterozygosity and Increase in Homozygosity

The population bottleneck also leads to an increased percentage of homozygous individuals in the population, as well as decreased heterozygosity. This loss of variation might lower resistance to environmental stressors and disease, which in turn could further endangered the population's survival.

Conclusion

Population bottlenecks influence the frequency of genetic diseases by reducing genetic diversity, increasing the chances of inbreeding, and resulting in higher percentages of homozygosity within the population. This leads to an increased likelihood of offspring carrying harmful alleles and developing genetic diseases.

Key concepts

Genetic Diversity

Genetic diversity is the total number of genetic characteristics within the genetic makeup of a species. It serves as a way for populations to adapt to changing environments. With more variation, it is more likely that some individuals in a population will possess genetic mutations that are suited to the environment and thus survive to pass these traits on to their offspring.

This concept is particularly important when we consider the implications of a population bottleneck. The drastic decrease in population size can significantly reduce genetic diversity. This loss means fewer unique traits within the population, which can result in decreased adaptability. With a narrow genetic pool, future generations are less equipped to handle changes in the environment—be it diseases, climate shifts, or other ecological changes.

Inbreeding

Inbreeding refers to the breeding of individuals with close genetic relationships. It is a direct consequence of a population bottleneck, where the reduced population size results in a limited pool of mates.

Why is inbreeding a concern?

It increases the probability of offspring being affected by recessive or deleterious traits. This happens because closely related individuals are more likely to carry the same harmful alleles, and when these individuals mate, their offspring is more likely to inherit two copies of these alleles, leading to genetic diseases or disorders.

Genetic Diseases

Genetic diseases are disorders caused by abnormalities in an individual's DNA. These abnormalities can range from a small mutation in a single gene to the addition or subtraction of an entire chromosome or set of chromosomes.

Following a population bottleneck, the likelihood of genetic diseases can increase because the genetic uniformity of a population tends to be higher. If the bottleneck leads to the survival of individuals who carry alleles for genetic diseases, these alleles can become more common in the population as the genetic pool is more restricted.

Loss of Heterozygosity

Heterozygosity refers to the presence of different alleles at a gene locus. A high level of heterozygosity typically indicates a lot of genetic diversity within a population. The loss of heterozygosity means there is a decrease in the number of individuals who have different alleles at a particular gene locus.

This loss is indicative of reduced genetic diversity and is often directly linked with the occurrence of a population bottleneck. With fewer diverse traits, a population may not only become more susceptible to genetic diseases but may also have reduced resilience against environmental pressures.

Homozygosity

Homozygosity is a term that describes a genetic condition where an individual has two of the same allele, whether dominant or recessive, at a genetic locus. High homozygosity can be an indication of inbreeding and is a common issue in populations that have experienced a bottleneck.

While homozygosity can sometimes be beneficial if it increases the proportion of individuals carrying advantageous traits, it's often associated with negative outcomes like increased prevalence of inherited diseases. This is because deleterious alleles can express themselves more freely in the absence of their dominant, healthier counterparts, potentially leading to health issues within the population.