Dihybrid Crosses
Learning Objective (DOT point)
Describe outcomes of dihybrid crosses involving
simple dominance using Mendel’s explanations
• Dihybrid crosses are those
where we consider the
inheritance of two
characteristics at the same
time.
Dihybrid Cross
• monohybrid = 1 trait with 2 alleles
= 1 allele gamete each for 2x2 box
• dihybrid = 2 traits with 2 alleles EACH = 4 alleles
= 2 allele gametes each for 4x4 box
[one allele for each trait in each gamete]
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…
FOIL method
• First, Outer, Inner, Last
• (ab) (cd)
•
•
•
•
First (ac)
Outer (ad)
Inner (bc)
Last (bd)
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
Dihybrid cross proportions
Phenotypes
Round Yellow
Proportions
9/16 or 56.25%
Round Green
3/16 or 18.75%
Wrinkled Yellow
3/16 or 18.75%
Wrinkled Green
1/16 or 6.25%
F2
9 : 3 : 3 : 1
Round
Round Wrinkled Wrinkled
Yellow
Green Yellow
Green
This is the typical ratio
expected in a dihybrid cross.