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
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chromosome. If these traits are more than 50 cM apart they will
behave as if they are assorting independently from one another.
RY
RY
Ry
rY
ry
Ry
rY
ry
RRYY RRYy RrYY
RrYy
RRYy RRyy
RrYy
Rryy
RrYY
RrYy
rrYY
rrYy
RrYy
Rryy
rrYy
rryy
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).
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