This document describes how to perform a dihybrid Punnett square cross to determine the possible offspring genotypes and phenotypes for two traits. It uses the example of crossing a heterozygous individual for both fur color (B/b) and coat texture (R/r) with another heterozygous individual. The steps involve determining the parental genotypes, writing out the possible gametes, setting up a Punnett square with the gametes, and filling in the possible offspring genotypes. The document then asks how many offspring would have each phenotype. It provides practice with another dihybrid cross in pea plants and assigns a worksheet for additional practice.

1. DIHYBRID PUNNETT SQUARES
https://www.youtube.com/watch?v=qIGXTJ
LrLf8

2. Dihybrid Cross:
a cross that shows the possible offspring for two traits
Fur Color:
B: Black
b: White
Coat Texture:
R: Rough
r: Smooth
Step 1: FIND OUT THE GENOTYPES OF THE PARENTS
In this example, we will cross a heterozygous
individual with another heterozygous individual.
Their genotypes will be:
BbRr x BbRr
Trait one Trait Two

3. Dihybrid Cross
BbRr x BbRr
Step 2:FOIL EACH PARENT TO SHOW THE
POSSIBLE GAMETE COMBINATIONS
Remember, each gamete must have one B and
one R.
DAD MOM

5. F.O.I.L.
BbRr
BR Br bR
br
PARENT 1 PARENT 2
BR Br
bR br
BbRr

6. Practice
What are the possible gametes for BBrr
What are the possible gametes for HhTT?

7. Dihybrid Cross
BbRr x BbRr
DAD MOM
Step 3:SET UP PUNNETT SQUARE:
Arrange all possible gametes for dad on
the top of your Punnett Square, and mom
down the side

8. Dihybrid Crosses:
a cross that shows the possible offspring for two traits
Fur Color:
B: Black
b: White
Coat Texture:
R: Rough
r: Smooth
BbRr x BbRr BR bR
br
bR
Br
BR
br
Br
Step 4: Fill in the
Punnett Square
(find the possible
genotypes of the
offspring)
B
B
B
B
B
B
B
B
b
b
b
b
b
b
b
b
B B B B
B B B B
b b b b
b b b b
R
R
R
R
r
r
r
r
R
R
R
R
r
r
r
r
R R R R
r r r r
R R R R
r r r r

9. BR bR
br
bR
Br
BR
br
Br
BBRR BbRR BbRr
BBRr BBrr BbRr Bbrr
BbRR BbRr bbRR bbRr
BbRr Bbrr bbRr bbrr
BBRr
How many of the
offspring would have a
black, rough coat?
How many of the
offspring would have a
black, smooth coat?
How many of the
offspring would have a
white, rough coat?
How many of the
offspring would have a
white, smooth coat?
Fur Color:
B: Black
b: White
Coat Texture:
R: Rough
r: Smooth

10. BR bR
br
bR
Br
BR
br
Br
BBRR BbRR BbRr
BBRr BBrr BbRr Bbrr
BbRR BbRr bbRR bbRr
BbRr Bbrr bbRr bbrr
BBRr
How many of the
offspring would have
black, rough coat?
How many of the
offspring would have a
black, smooth coat?
How many of the
offspring would have a
white, rough coat?
How many of the
offspring would have a
white, smooth coat?
Fur Color:
B: Black
b: White
Coat Texture:
R: Rough
r: Smooth
Phenotypic Ratio=
9:3:3:1

11. Practice in your notes…
In pea plants, yellow seeds (Y) are dominant over green
seeds (y), and rounded peas (R) are dominant over
wrinkled peas (r).
Cross a plant that is heterozygous for both traits with a
plant that is homozygous recessive for both traits.
Draw a Punnett square to show all possible offspring,
and determine the genotypic and phenotypic ratios.

12. YyRr X yyrr
YR yR
yr
yr
yr
yr
yr
Yr
YyRr yyRr yyrr
YyRr Yyrr yyRr yyrr
YyRr Yyrr yyRr yyrr
YyRr Yyrr yyRr yyrr
Yyrr

13. More Practice
• Work on the SpongeBob Dihybrid Cross
worksheet. Turn it in at the end of class.