Dihybrid Crosses
Multiple trait inheritance
Mendel’s monohybrid crosses were based on one characteristic controlled by one gene such as seed
shape (round or wrinkled) in a pea plant. He then went on to ask: what happens when more than one
characteristic is involved in a cross?
He focused on two characteristics:
i.
Seed shape - (Round - R, and Wrinkled – r)
ii.
Seed colour - (Yellow - Y, and Green - y)
He crosses two plants that were heterozygous for seed shape (Rr) and seed colour (Yy) in what we now
know as a Dihybrid Cross.
The first figure shows a cross between to homozygous parent plants that produce plants that are
heterozygous for these two traits.
Round, Yellow seed
RRYY
Parent Genotypes
Alleles in
gametes
rryy
ry
RY
RY
RrYy
RrYy
RrYy
RrYy
Wrinkled Green seed
ry
A heterozygous individual for two characteristics will produce four possible gametes. The figure below
demonstrates the possible combinations:
R
RY
Ry
r
Y
y
rY
ry
A parent that is RrYy can produce the following gametes: RY, Ry, rY, ry.
The law of independent assortment: states that each allele is independent of the other and no two
alleles are linked. In other words, the inheritance of seed shape has no influence on the inheritance of
seed colour. A dihybryd cross can be treated as two separate monohybrid crosses.
The following a what Mendel’s cross between to heterozygous Round, Yellow seeds looks like:
Round, Yellow seed
RrYy
Parent Genotypes
Alleles in
gametes
RY
Ry
Round, Yellow seed
RrYy
rY
ry
F2 genotype:
F2 phenotype:
RY
Ry
rY
ry