Linkage equilibrium

Linkage Equilibrium is a fundamental concept in the field of genetics, particularly within the study of population genetics. It describes the situation in which the alleles at two different genes or loci are associated with each other within a population at frequencies that are the product of their individual frequencies. This concept is crucial for understanding the genetic structure of populations and the forces that shape genetic variation.

Definition

Linkage equilibrium occurs when the genotype at one locus is independent of the genotype at another locus across the individuals in a population. In other words, the occurrence of an allele at one locus does not affect the occurrence of an allele at another locus. This situation is opposite to linkage disequilibrium, where there is a non-random association between alleles at different loci.

Mathematical Representation

The condition for linkage equilibrium can be mathematically represented for two loci, A and B, with alleles A1, A2 and B1, B2, respectively. If the frequencies of these alleles in a population are p(A1), p(A2), p(B1), and p(B2), and the frequency of a haplotype (combination of alleles at different loci) A1B1 is represented as p(A1B1), then linkage equilibrium is said to occur if:

\[ p(A1B1) = p(A1) \times p(B1) \]

Similarly, this condition must hold true for all combinations of alleles at the two loci.

Factors Influencing Linkage Equilibrium

Several factors can influence whether a population is in linkage equilibrium, including:

• Mutation: New mutations can alter allele frequencies, potentially disrupting existing equilibria.
• Genetic drift: Random changes in allele frequencies can lead to changes in the association between alleles at different loci.
• Gene flow: The introduction of alleles from other populations can alter the genetic structure and associations within a population.
• Selection: Natural or artificial selection can favor certain allele combinations, leading to linkage disequilibrium.
• Recombination: The process by which alleles are reshuffled during meiosis can break down associations between alleles, pushing the population towards linkage equilibrium.

Importance in Genetics

Understanding linkage equilibrium is vital for geneticists and evolutionary biologists. It underpins the assumptions of many population genetics models and is crucial for the study of genetic linkage, genetic mapping, and the identification of genetic markers. In medical genetics, linkage equilibrium and disequilibrium analyses are used to identify genes associated with diseases.

See Also

• Genetic linkage
• Hardy-Weinberg principle
• Population genetics
• Quantitative genetics