Dihybrid Crosses
Crosses involving two traits
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
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
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 1
Gamete 2
Gamete 3
Gamete 4
Gamete 2
Gamete 3
Gamete 4
Combining gametes
 List the gametes for Parent 1
along one edge of the Punnett
YR
Yr
yR
yr
square
 List the gametes for Parent 2 YR YYRR YYRr YyRR YyRr
along the other edge of the
Punnett square
Yr YYRr YYrr YyRr Yyrr
 Fill out the squares with alleles
from both parents (keeping
similar letters together)
yR YyRR YyRr yyRR yyRr
 The result is the prediction of
all possible combinations of
YyRr Yyrr yyRr yyrr
yr
genotypes for the offspring of
the dihybrid cross
Calculating Ratios
 Yellow & Round (Y_R_): 9
 green & Round (yyR_): 3
 Yellow & wrinkled (Y_rr): 3
 green & wrinkled (yyrr): 1
YR
Yr
yR
yr
YR YYRR YYRr YyRR YyRr
Yr
 The phenotypic result of 2
heterozygotes in a dihybrid
cross will always be 9:3:3:1 (2
dom; 1dom/1rec; 1rec/1dom; yR
2 rec)
yr
 The genotypic result is a bit
more complex
YYRr YYrr
YyRr Yyrr
YyRR YyRr yyRR yyRr
YyRr Yyrr
yyRr
yyrr
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:
1. Tall, Purple
8/16 = 50%
2. Tall, White
0/16 = 0%
3. Short, Purple
8/16 = 50%
4. Short, White
0/16 = 0%
Review Questions
1. Differentiate between monohybrid and dihybrid crosses.
2. Explain the four basic steps to solving a dihybrid cross.
3. Explain how to determine the alleles for each parent that are part of
the Punnett square.
4. Explain how to combine gametes and alleles in a dihybrid Punnett
square.
5. Use a Punnett square to solve dihybrid genetics crosses.