Final answer:
Linkage equilibrium occurs when alleles at different loci are recombined randomly, with no preferential association, whereas linkage disequilibrium refers to the non-random association between alleles, often due to their physical proximity on a chromosome. Crossing over can influence genetic variation by disrupting linkage disequilibrium.
Step-by-step explanation:
Linkage equilibrium and linkage disequilibrium refer to the genetic relationships between alleles at different loci. Linkage equilibrium exists when the genotype of a chromosome at one locus is independent of its genotype at another locus; the alleles on different loci are recombined according to the expected frequencies if they are assorted randomly. This state suggests there's no preferential association between certain alleles at different loci. Conversely, linkage disequilibrium occurs when alleles at different loci are associated together more or less frequently than expected by chance. This can happen due to physical proximity on a chromosome, preventing independent assortment during meiosis. Such non-random association between alleles signifies that they are linked. Genetic variation can be influenced by the process of crossing over during meiosis, which can disrupt existing linkage disequilibrium by exchanging alleles between homologous chromosomes, leading to new combinations of alleles. A high frequency of crossing over can lead to a more randomized assortment and move closer towards linkage equilibrium. Linkage disequilibrium is important in understanding the principles that govern the inheritance of complex traits and in mapping genetic diseases.