Recombination is the process whereby a piece of DNA or RNA is broken from its parent molecule and inserted into a new position in a new molecule. Recombination plays a role in many processes, including DNA repair, but notably in chromosomal crossover during Prophase I of meiosis where it is specifically termed homologous recombination. During this period, homologous chromosomes are closely associated and at sites called chaisma and a piece of one chromosome may break off and be integrated into its homologous partner. The probability of a recombination event (recombination frequency) is equally small at any position on the chromosome, so the farther apart two loci are, the greater the likelihood of the loci being separated by a recombination event. A gamete that contains chromosomes which are distinct from its parental chromosomes due to recombination of alleles is called a recombinant gamete and such a gamete will give rise to a recombinant offspring.

Although chromosomal crossing over usually occurs between matched sequences, there may rarely be a slight mismatch due to similarities in sequence. This is called unbalanced recombination and if a gamete containing a chromosome that is an unbalanced recombinant forms a zygote, then there may be serious consequences including the deletion, duplication or inversion of the zygote's genetic material.