Extended Haplotype Homozygosity (EHH)

Definition

Extended haplotype homozygosity (EHH) refers to the phenomenon where a specific haplotype, or a combination of alleles at adjacent loci, is found to be identical across a significant region of the genome within a population. This pattern often indicates that a region has undergone selective pressure, either positive or negative, which can be crucial for understanding how certain genes are maintained or eliminated in populations over time.

5 Must Know Facts For Your Next Test

1. EHH is used as a measure of how much genetic variation remains in the vicinity of a selected allele, helping to identify regions of the genome that may be subject to selection.
2. Positive selection leads to increased EHH values, indicating that advantageous alleles are being retained across longer genomic distances.
3. Negative selection typically results in lower EHH values, as deleterious alleles tend to be removed from the population more quickly, reducing haplotype diversity.
4. EHH can provide insights into population history and structure by revealing patterns of selection and gene flow over time.
5. Analyses of EHH can be applied in various fields, such as conservation genetics and understanding the genetic basis of complex diseases.

Review Questions

• How does extended haplotype homozygosity (EHH) reflect the effects of positive selection on a population's genetic diversity?
  • Extended haplotype homozygosity (EHH) reflects positive selection by showing increased levels of homozygosity for specific haplotypes across extended genomic regions. When an advantageous mutation occurs, it can spread through the population, causing linked alleles in the surrounding area to also increase in frequency. This results in less variation around the selected allele, indicating that natural selection is favoring certain genetic combinations and reducing overall genetic diversity in that region.
• Discuss the implications of EHH patterns in understanding negative selection and its effects on genetic variation within populations.
  • Patterns of extended haplotype homozygosity (EHH) can provide insight into negative selection by revealing areas with reduced haplotype diversity. When deleterious alleles are present, negative selection acts to eliminate these alleles from the population. Consequently, the EHH values tend to be lower, demonstrating that there is less homozygosity for those regions. This reduction in genetic variation indicates that harmful alleles are being efficiently purged from the gene pool, which is vital for maintaining the overall fitness of the population.
• Evaluate how understanding extended haplotype homozygosity (EHH) can inform research on human adaptation and disease susceptibility.
  • Understanding extended haplotype homozygosity (EHH) is crucial for evaluating human adaptation and disease susceptibility because it reveals how populations respond to environmental pressures and genetic factors over time. By identifying regions with high EHH values linked to beneficial traits, researchers can pinpoint genetic variants that contribute to adaptive responses. Conversely, low EHH values may highlight areas where harmful mutations are effectively removed. This knowledge helps in understanding not only historical adaptations but also informs current research on susceptibility to complex diseases influenced by these evolutionary dynamics.

Related terms

Haplotype: A combination of alleles at multiple loci on a chromosome that are inherited together.

Selective Sweeps: The process by which a beneficial mutation becomes more frequent in a population, often leading to reduced genetic variation around that locus.

Genetic Drift: The random fluctuations in allele frequencies within a population, which can lead to the loss of genetic variation over time.