1. Crosses involving two traits
Dihybrid Crosses
Crosses involving two traits

2. Dihybrid Crosses
Constructing a Punnett square for a dihybrid cross is similar to the method used for a monohybrid cross
First, determine the alleles produced in the gametes of each parent
Next, construct a Punnett square
Then, combine the gametes for each cell
Finally, calculate genotypic or phenotypic ratios

3. Determining alleles
In Mendel’s peas, let’s consider pea color and pea shape
Y = yellow, y = green
R = round, r = wrinkled
First determine the genotype for the 2 traits of the individual (YYRR, yyrr, YyRr, etc…)
The “FOIL” method (from Algebra) may be helpful in determining allele possibilities for an individual
(YY)(RR) = YR, YR, YR, YR
(yy)(rr) = yr, yr, yr, yr
(Yy)(Rr) = YR, Yr, yR, yR
Let’s use a YyRr x YyRr cross since it is the most complex

4. Constructing a Punnett square
Mendel’s laws support the creation of four combinations of gametes for each individual
Remember that meiosis produces 4 unique daughter cells
Since each parent produces 4 different combinations of alleles in the gametes, draw a 4 square by 4 square Punnett square
Gamete 1
Gamete 2
Gamete 3
Gamete 4

5. Combining gametes
List the gametes for Parent 1 along one edge of the Punnett square
List the gametes for Parent 2 along the other edge of the Punnett square
Fill out the squares with alleles from both parents (keeping similar letters together)
The result is the prediction of all possible combinations of genotypes for the offspring of the dihybrid cross YR Yr yR yr YR
YYRR
YYRr
YyRR
YyRr Yr
YYRr
YYrr
YyRr
Yyrr yR
YyRR
YyRr
yyRR
yyRr yr
YyRr
Yyrr
yyRr
yyrr

6. Calculating Ratios Yellow & Round (Y_R_): 9 green & Round (yyR_): 3
Yellow & wrinkled (Y_rr): 3
green & wrinkled (yyrr): 1
The phenotypic result of 2 heterozygotes in a dihybrid cross will always be 9:3:3:1 (2 dom; 1dom/1rec; 1rec/1dom; 2 rec)
The genotypic result is a bit more complex YR Yr yR yr YR
YYRR
YYRr
YyRR
YyRr Yr
YYRr
YYrr
YyRr
Yyrr yR
YyRR
YyRr
yyRR
yyRr yr
YyRr
Yyrr
yyRr
yyrr

7. Practice!!!
In pea plants, purple flowers (F) are dominant to white flowers (f) and tall stems (T) are dominant to short stems (t). If a tall (heterozygous), purple (homozygous) flowered pea plant is crossed with a short, white flowered pea plant. What are the possibilities of having offspring of:
Tall, Purple
Tall, White
Short, Purple
Short, White
8/16 = 50%
0/16 = 0%
8/16 = 50%
0/16 = 0%

8. Review Questions
Differentiate between monohybrid and dihybrid crosses.
Explain the four basic steps to solving a dihybrid cross.
Explain how to determine the alleles for each parent that are part of the Punnett square.
Explain how to combine gametes and alleles in a dihybrid Punnett square.
Use a Punnett square to solve dihybrid genetics crosses.