The process of meiosis leading to a separation of chromosomes, as well as crossing over, is necessary for the understanding of the process of recombination.

The term “recombination” is used in several different contexts in genetics. In reference to heredity, recombination is defined as a process that results in gametes with combinations of alleles that were not present in the gametes from the parental generation (Figure 9.3.1). Recombination is important because it contributes to the genetic variation that may be observed between individuals within a population and that may be acted upon by selection for evolution.

Watch video, Genetic recombination 1 | Biomolecules | MCAT | Khan Academy Medicine, created by Efrat Bruck (2015) at Khan Academy on YouTube (https://www.youtube.com/watch?v=BlnUNmfGn7I).

Inter- and Intrachromosomal Recombination

Interchromosomal recombination occurs either through independent assortment of alleles whose loci are on different chromosomes. Intrachromosomal recombination occurs through crossovers between loci on the same chromosomes. It is important to remember that in both of these cases, recombination is a process that occurs during meiosis (mitotic recombination may also occur in some species, but it is relatively rare).

As an example of interchromosomal recombination, consider loci on two different chromosomes as shown in Figure 9.3.1 We know that if these loci are on different chromosomes there is no physical connection between them, so they are unlinked and will segregate independently as did Mendel’s traits. The segregation depends on the relative orientation of each pair of chromosomes at metaphase. Since the orientation is random and independent of other chromosomes, each of the arrangements (and their meiotic products) is equally possible for two unlinked loci as shown in Figure 9.3.1.

Intrachromosomal recombination occurs through crossovers. Crossovers occur during prophase I of meiosis, when pairs of homologous chromosomes have aligned with each other in a process called synapsis. Crossing over begins with the breakage of DNA of a pair of non-sister chromatids. The breaks occur at corresponding positions on two non-sister chromatids, and then the ends of non-sister chromatids are connected to each other resulting in a reciprocal exchange of double-stranded DNA. Generally, every pair of chromosomes has at least one crossover during meiosis, but often multiple crossovers occur in each chromatid during prophase I.

Because interchromosomal recombination occurs through independent assortment, genes in this situation are always unlinked. Intrachromosomal recombination has instances of linked genes, and so they will be the focus of this chapter.

Inheriting Parental and Recombinant Gametes

If we consider only two loci and the products of meiosis result in recombination, then the meiotic products (gametes) are said to have a recombinant genotype. On the other hand, if no recombination occurs between the two loci during meiosis, the products retain their original combinations and are said to have a non-recombinant, or parental genotype. The ability to properly identify parental and recombinant gametes is essential to apply recombination to experimental examples.

To properly identify recombinant and parental gametes from an individual, you need to know the genotype of its parents (the P generation). This is most easily demonstrated in a dihybrid. If, for two genes, one parent has the genotype A/A B/B, they can only produce one type of gamete: AB.  Similarly, if they are a/a b/b, they can also only produce one type of gamete: ab (Figure 9.3.2 right). However, if those two gametes (AB and ab) combine, they create an individual (F1) with a genotype written as A/a B/b. It can be easier to keep track of the parental combinations of gametes by keeping them together when writing the genotype, for this example AB/ab (Figure 9.3.2).

So, the above dihybrid individual can produce four different gametes: AB, ab, Ab and aB. The parental gametes are those that the individual obtained from their parents, in this case AB and ab. Ab and aB are recombinant gametes and are evidence of a recombination event happening, resulting in a different combination of alleles (Figure 9.3.2 right).

For the above example, the P generation has one parent homozygous for both dominant alleles, and the other homozygous for both recessive alleles. It is important to note that this will not always be the case. In some instances, one parent will be homozygous, with one gene dominant and the other gene recessive (A/A b/b), and the other parent will be the opposite (a/a B/B). This situation will change, which is the parental and recombinant gametes (compare left and right in Figure 9.3.2).