Monday, January 10, 2011


Objective: SWBAT complete Dihybrid Punnett
squares.
Bellringer: Use a Punnett square to predict the
offspring of parents that are hybrid dominant and
homozygous recessive for the trait of Furriness (use
the letter F).
Agenda
1.
2.
3.
4.
Bellringer
Guided notes: Dihybrid Crosses
Practice problems
Homework: SpongeBob Dihybrid Crosses
DIHYBRID PUNNETT
SQUARES
Mastering the Dreaded Dihybrid
What’s the difference?
Monohybrid cross

When only ONE trait is
looked at


Dihybrid cross

For example, height OR
color
Use a 2x2 Punnett
square
When TWO traits are
looked at


For example, height AND
color
Use a 4x4 Punnett
square
Steps to complete a Dihybrid Cross
Note: These are the same as for a Monohybrid Cross, but now
you’re doing them for two genes rather than just one!
1.
2.
Determine the genotypes of the parents.
Figure out the possible gamete combinations for
each parent. (gametes = sex cells)


3.
Each gamete gets one copy of each gene.
Helpful Hint: Use the FOIL method to make pairs.
Complete the 4x4 Punnett square.

Helpful Hint: Complete one gene (one letter type) and
then the second gene (the second letter).
Dihybrid Example
Step by Step
An Example Problem
What are the possible genotypes and
phenotypes of the children of parents that
are heterozygous for round eyes with a
blue body and purebred for round eyes
and hybrid for a yellow body?
Step 1: Determine the genotypes of
the parents.
Parent 1:


heterozygous for round
eyes
blue body
Parent 2:


purebred for round
eyes
hybrid for a yellow
body
Step 2: Figure out the possible gamete combinations
for each parent. (gametes = sex cells)
•
•
Each gamete gets one copy of each gene.
Helpful Hint: Use the FOIL method to make pairs.
• FOIL pairs = First, Outer, Inner, Last
Parent 1:
Parent 2:
Gametes:
Gametes:
Ry
Ry
ry
ry
RY
Ry
RY
First
Outer
Inner
Last
First
Outer
Inner
Ry
Last
Step 3: Complete the 4x4 Punnett square.
Helpful Hint: Complete one gene (one letter type) and then the
second gene (the second letter).
Parent 1:
Parent 2:
Gametes:
Ry
Ry
Gametes:
ry
RY
ry
Ry
Ry
Ry
RY
ry
ry
RY
R R Y y R R Y y
R r Y y
R r Y y
Ry
R R y y R R y y
R r y y
R r y y
RY
R R Y y R R Y y
R r Y y
R r Y y
Ry
R R y y R R y y
R r y y
R r y y
Ry
Genotype & Phenotype Ratios
Ry

ry
ry
RY
R R Y y R R Y y
R r Y y
R r Y y
Ry
R R y y R R y y
R r y y
R r y y
RY
R R Y y R R Y y
R r Y y
R r Y y
Ry
R R y y R R y y
R r y y
R r y y
Genotypes:
4

Ry
RRYy : 4 RrYy : 4 RRyy : 4 Rryy
Phenotypes:

8 Round-eyed, yellow body : 8 Round-eyed, blue body
Homework
Complete the practice problems from the notes.
 Complete the Bikini Bottom Dihybrid Practice
problem set.


If you didn’t yet take last Friday’s QUIZ, please
come see me in room 109 after school or during
your lunch period to take the Quiz. (We will go
over the Quiz tomorrow.)