How does chance affect combinations of genes?

During crossing over or genetic recombination (also known as chiasma), which occurs during prophase I of meiosis, the pair of matched chromosomes (from two parents) undergo a process known as synapsis, during which genes are exchanged. The assumption is that the process is random, although the likelihood that two genes will be combined together is inversely related to their distances from each other on the chromosome, a concept known as genetic linkage.

Non-randomness could be said to exist in when thee is linkage disequilibrium, a situation in which some combinations of genes or genetic markers occur more or less frequently in a population than would be expected from their distances apart.

Although crossovers typically occur between homologous regions of matching chromosomes, similarities in sequence can result in mismatched alignments. Such unbalanced recombination is often lethal, and severe problems can arise if a gamete containing unbalanced recombinants becomes part of a zygote. The result can be a local duplication of genes on one chromosome and a deletion of these on the other, a translocation of part of one chromosome onto a different one, or an inversion.