Transcript This is the phenotypic ratio expected in a dihybrid cross.

Dihybrid Crosses
• 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 color 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, round and yellow. As with
monohybrid crosses, he then crossed two of the F1
generation together…
Phenotype
Genotype
Gametes
F2
Round, yellow
RrYy
RY, Ry, rY, ry
X
Round, yellow
RrYy
RY, Ry, rY, ry
We really need a punnet square for
this…
RY
Ry
rY
ry
RY
RRYY
RRYy
RrYY
RrYy
Ry
RRYy
RRyy
RrYy
Rryy
rY
RrYY
RrYy
rrYY
rrYy
ry
RrYy
Rryy
rrYy
rryy
F2
9 : 3 : 3 : 1
Round
Round
Wrinkled Wrinkled
Yellow
Green
Yellow
Green
This is the phenotypic ratio
expected in a dihybrid cross.
• 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