1. Dihybrid Crosses

2. Two characteristics at once…
Dihybrid crosses are where we consider heredity in two characteristics at the same time.
There are four possible combinations of two characteristics. Example: Pea color and shape. Peas can be...
1) round and yellow
2) round and green
3) wrinkled and yellow
4) wrinkled and green

3. X P generation cross… rryy RRYY Phenotype Round, yellow
Wrinkled, green
rryy
RRYY
Genotype
all RY
all ry
Gametes
all RrYy F1
All in F1 generation are heterozygous for both characteristics (all round and yellow)

4. X F1 generation cross… RrYy RrYy RY, Ry, rY, ry RY, Ry, rY, ry
Phenotype
Round, yellow
Round, yellow
RrYy
RrYy
Genotype
RY, Ry, rY, ry
RY, Ry, rY, ry
Gametes F2
We really need a punnet square for this…

5. RY Ry rY ry
RRYY
RRYy
RrYY
RrYy
RRyy
Rryy
rrYY
rrYy
rryy

6. This is the typical ratio expected in an F1 dihybrid cross
F2 generation ratio : : : 9 3 3 1
Round
Yellow
Round
Green
Wrinkled
Yellow
Wrinkled
Green
This is the typical ratio expected in an F1 dihybrid cross

7. Now for trihybrid Crosses...?