DIHYBRID CROSSES
A dihybrid cross involves two alleles per trait for two traits, for a total of four alleles.

Gamete formation must occur for guinea pigs and other sexually reproducing organisms to reproduce. Thus, the first step to solving a dihybrid cross is to see how the various alleles (forms of the two traits) combine in the gametes. The principle of independent assortment explains this combining of alleles.

Independent assortment means that each allele of a particular trait has an equal chance of being in a gamete with each of the alleles of the other trait; that is, B has an equal chance of ending up with S or s during gamete formation. This is allele pairing—alleles for the different traits pair. Independent assortment occurs during meiosis.

For example, the male guinea pig's alleles for this dihybrid cross are BbSs. The possible allele combinations in the male guinea pig's gametes are:

In this example, the same possible allele combinations apply to the female, and to both guinea pigs' gametes.

F₁ guinea pigs are crossed (mated). These guinea pigs are heterozygous for black, short hair (BbSs).

Next, all possible gamete combinations (with their alleles) from the two F₁ guinea pigs must be considered. A Punnett square is an easy way to determine these combinations.