∞ generated and posted on 2016.08.28 ∞

Close physical proximity especially of loci on chromosomes.

Linkage is when two genes are difficult to separate, such as because they are so close together.

Linkage can be inferred from base sequence information but is demonstrated only through frequency of recombination determinations. In particular, degree of linkage can vary between loci, ranging from tightly linked (very close proximity; frequency of recombination <<<50%) to unlinked (frequency of recombination = 50%).

The concept of linkage is quite relevant to the evolution of gene complexes, where greater linkage can allow for greater coevolution among genes. For asexual organisms, all of the genes may be essentially linked even if they are not found on the same chromosomes since there is no recombination to test that linkage (or lack of linkage). One can even speak of a linkage between nuclear genes and those genes that are cytoplasmically inherited, i.e., as associated with endosymbiotic bacteria.

See also linkage equilibrium and linkage disequilibrium.

During horizontal gene transfer, those genes that are more closely linked will with greater likelihood be transferred at the same time. This is particularly relevant for genes that are functional only as part of groups of genes, such as an operon, since movement of such genes will be selectively ineffectual without movement also of the associated genes.

The following video considers both the various situations associated with Mendelian genetics as a applied to circumstances in which more than one locus is followed: