A Dihybrid Cross Example Using Mendel’s Sweet Peas
Because a hybrid cross refers to a cross between heterozygous individuals (and di- refers to two traits), a dihybrid cross refers to a cross between two individuals that are heterozygous for two traits. In the following example, we will examine a dihybrid cross for two traits found in pea plants: pea shape and pea color. Gregor Mendel used these traits in his work. The rule for pea shape is that round is completely dominant to wrinkled, and the rule for pea color is that green is completely dominant to yellow; thus, homozygous-dominant and heterozygous individuals will have round green peas.
Homozygous recessive individuals will have wrinkled yellow peas. Individuals can have a mixture of wrinkled green peas and smooth yellow peas depending on their genotype.
Our dihybrid cross example:
RrYy x RrYy is a dihybrid cross.
The shape of the pea is controlled by one set of alleles, where round is completely dominant to wrinkled:
RR = round
Rr = round rr = wrinkled
The second set of alleles in this example controls the color of the peas. Green is dominant to yellow:
YY = green
Yy = green yy = yellow
The gametes for each individual could be either RY, Ry, rY, and Ry, with a 25% chance of any of them. In our example, these two sets of alleles are assorting independently
(according to the laws of independent assortment). Either they are on independent chromosomes or they are more than 50 cM apart on the same chromosome. If these traits are more than 50 cM apart they will behave as if they are assorting independently from one another.

In this case, the genotypic ration is 1 (RRYY): 2(RRYy): 2(RrYY): 1(RRyy): 4 (RrYy):
2(Rryy): 1 (rrYY): 2(rrY): 1(rryy).
Because round is completely dominant to wrinkled and green is completely dominant to yellow, the phenotypic ratio is 9 (round green): 3 (round yellow): 3 (wrinkled green): 1
(wrinkled yellow).