Dihybrid Crosses
A2 Biology
Meiosis, genetics & gene control
A Graham
Carmel RC Technology College
Darlington
• Dihybrid crosses are those
where we consider the
inheritance of two
characteristics at the same
time.
Mendel’s Peas (again!)
Mendel had observed that the seeds of his
pea plants varied in several ways – among
them, whether the peas were round or
wrinkled and whether they were green or
yellow.
He did monohybrid crosses on both
characteristics, but then decided to look at
how they were inherited together….
• There are four possible combinations of
the two characteristics. Peas can be:
• 1) round and yellow
• 2) round and green
• 3) wrinkled and yellow
• 4) wrinkled and green
• From his monohybrid trials, Mendel had
discovered that the allele for yellow colour
was dominant to that for green.
• He had also found that the allele for round
peas was dominant to the allele for
wrinkled peas.
• Starting with individuals that were
homozygous, either for both dominant
characteristics or both recessive
characteristics, he set off breeding again…
Phenotype
Round, yellow
Genotype
RRYY
Gametes
all
F1
X
rryy
RY
all
Wrinkled, green
all
ry
RrYy
All of the F1 generation would be heterozygous for both
characteristics, meaning that they would all be round and
yellow. As with monohybrid crosses, he then crossed two
of the F1 generation together…
Phenotype
Round, yellow
Genotype
RrYy
Gametes
RY, Ry, rY, ry
F2
X
Round, yellow
RrYy
RY, Ry, rY, ry
We really need a punnet square for this…
RY
¼
Ry
¼
rY
¼
ry
¼
RY
¼
RRYY
RRYy
RrYY
RrYy
Ry
¼
RRYy
RRyy
RrYy
Rryy
rY
¼
RrYY
RrYy
rrYY
rrYy
ry
¼
RrYy
Rryy
rrYy
rryy
F2
9 : 3 : 3 : 1
Round
Round Wrinkled Wrinkled
Yellow
Green Yellow
Green
This is the typical ratio
expected in a dihybrid cross.