A dihybrid cross involves a study of inheritance patterns for organisms differing in two traits. Consider Mendel's experiments with pea
plants. If we look at two traits of Mendel's peas, say seed shape and seed color, we will see that these two traits are inherited
independently.
A pea plant is heterozygous for both seed shape and seed color. S is the allele for the dominant, spherical shape characteristics is the
allele for the recessive, dented shape characteristic. Y is the allele for the dominant, yellow color characteristic; y is the allele for the
recessive, green color characteristic.
Since this is a dihybrid cross, the genotype for each parent is SsYy.
Can you determine which statements are correct regarding a dihybrid cross between SsYy X S5Yy?
-The distribution of the two alleles in each parent plant's gametes will be 50% of gametes are sy; 50% of
gametes are SY.
-A phenotype ratio of 9:3:3:1 in the offspring of a mating of two organisms heterozygous for two traits is expected when the gene pairs assort independently during meiosis.
-Following a SsYy x SsYy cross, 3/4 of the offspring will be heterozygous for both traits.
-The gametes of a plant of genotype SsYy should have the -genotypes SY, Sy, sy, and sy.
In a dihybrid cross, 1/16 of the offspring will be homozygous for both recessive traits.

In this dihybrid cross, each gene locus had an independent effect on a single phenotype. Thus, the R and r alleles affected only the shape of the seed and had no influence on seed color, while the Y and y alleles affected only seed color and had no influence on seed shape. In this case, there were two separate genes that coded for two separate characteristics.

But what happens when two different loci affect the same characteristic? For instance, what if both of the loci in Mendel's experiment affected seed color? When two genes are involved in the outcome of one characteristic, a dihybrid cross involving these genes can produce a phenotypic ratio very different from 9:3:3:1. Under these circumstances, there are more than two gene products affecting the same phenotype, and these products may have complex hierarchical relationships. Any time two different genes contribute to a single phenotype and their effects are not merely additive, those genes are said to be epistatic.

Although some researchers have attempted to categorize all digenic (two-gene) epistatic interactions with specific names, those classification schemes are seldom used today. One reason that they have fallen out of favor is that terms such as "dominant" and "recessive" are best used to describe the effects of alleles of single genes. Furthermore, epitasis is not restricted to the interactions of only two genes. Rather, epistasis occurs in all of the following scenarios:

Whenever two or more loci interact to create new phenotypes

Whenever an allele at one locus masks the effects of alleles at one or more other loci

Whenever an allele at one locus modifies the effects of alleles at one or more other loci

Epistasis is an interaction at the phenotypic level of organization. The genes that are involved in a specific epistatic interaction may still show independent assortment at the genotypic level. In such cases, however, the phenotypic ratios may appear to deviate from those expected with independent assortment.

Epistatic Relationships Involving Two Genes

As previously mentioned, scientists have performed numerous studies in an attempt to better understand and classify digenic epistatic relationships. Some of the most famous examples of research in which the interaction between two genes was found to produce a novel phenotype are examined in the following sections.