A dihybrid cross in genetics refers to a breeding experiment involving two individuals that differ in alleles at two distinct genetic loci.

More specifically, a dihybrid cross is conducted between individuals that vary in two traits. The term “dihybrid” indicates that the parent organisms in the initial cross (known as the P generation) are heterozygous for both traits. For instance, when examining traits such as seed shape (either round or wrinkled) and seed color (yellow or green) in pea plants, a typical dihybrid cross would involve plants that are heterozygous for both traits, represented as $RrYy$.

Interpreting a dihybrid cross involves predicting the genotypes and phenotypes of the offspring in the F1 and F2 generations, based on Mendelian genetics principles. This includes the law of segregation, which asserts that the two alleles for each trait separate during gamete formation, and the law of independent assortment, which states that alleles for different traits assort independently of one another.

To facilitate the interpretation of a dihybrid cross, a Punnett square is commonly employed. This graphical tool illustrates all potential genetic combinations among the offspring. For a dihybrid cross, a $4 \times 4$ Punnett square is utilized, yielding a total of $16$ possible combinations. The proportions of these combinations can then be used to predict the phenotypic ratios of the offspring. For example, in a dihybrid cross between two heterozygous parents ($RrYy \times RrYy$), the expected phenotypic ratio is $9:3:3:1$, which corresponds to $9$ round yellow, $3$ wrinkled yellow, $3$ round green, and $1$ wrinkled green.

In summary, interpreting a dihybrid cross requires a solid understanding of the principles of Mendelian genetics and the application of a Punnett square to predict the genotypes and phenotypes of the offspring. This concept is fundamental in genetics and provides insight into how traits are inherited and how genetic variation emerges.