What is the phenomenon called when alleles situated in close proximity on a chromosome associate more than expected from individual allelic frequencies?

Linkage disequilibrium refers to the non-random association of alleles that are located close to one another on the same chromosome. When alleles are in linkage disequilibrium, they occur together in a population more frequently than would be expected based purely on their individual allele frequencies. This is often due to the physical proximity of the alleles on the chromosome, which reduces the likelihood of recombination occurring between them during meiosis.

In contrast, crossing over refers to the exchange of genetic material between homologous chromosomes and is a process that can disrupt linkage disequilibrium. Polymorphism describes the presence of two or more variants (alleles) at a particular genetic locus in a population but does not specifically address their association. Chimerism involves the presence of two genetically distinct cell lines within an individual, which is unrelated to the concept of allele association on chromosomes. Understanding these concepts is crucial in genetics, especially in studies related to inheritance patterns and population genetics.