Bell Work
• In pigeons, the allele B produces ash-red
feathers. The allele b produces blue
feathers. The B allele is dominant to the b
allele.
• A pigeon with genotype Bb is crossed with
a pigeon with genotype bb. What percent
of the offspring are expected to have ashred feathers?
• Justify your answer. You do not need
to copy the question.
Dihybrid Cross:
a cross that shows the possible
offspring for two traits
Fur Color:
B: Black
b: White
Coat Texture:
R: Rough
r: Smooth
In this example, we will cross a
heterozygous individual with another
heterozygous individual. Their
genotypes will be:
BbRr x BbRr
Dihybrid Cross
BbRr x BbRr
Step 1: Find ALL possible gametes that can
be made from each parent.
Remember, each gamete must have one B
and one R.
Dihybrid Cross
BbRr x BbRr
Possible gametes:
BR
Step 2: Arrange all possible
Br
gametes for one parent on
the top of your Punnett
bR
Square and the other parent
br
on the side
Dihybrid Crosses:
a cross that shows the possible offspring for two traits
BbRr x BbRr
Fur Color:
B: Black
b: White
Coat Texture:
R: Rough
r: Smooth
Step 3: Fill in the
Punnett Square
(find the possible
genotypes of the
offspring)
BR
BR
Br
bR
br
Br
bR
br
Dihybrid Crosses:
a cross that shows the possible offspring for two traits
BR
Br
bR
br
BR
BBRR
BBRr
BbRR
BbRr
Br
BBRr
BBrr
BbRr
Bbrr
bR
BbRR
BbRr
bbRR
bbRr
br
BbRr
Bbrr
bbRr
bbrr
BbRr x BbRr
Fur Color:
B: Black
b: White
Coat Texture:
R: Rough
r: Smooth
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?
BR
Br
bR
br
BR BBRR
BBRr
BbRR
BbRr
Br
BBRr
BBrr
BbRr
Bbrr
bR
BbRR
BbRr
bbRR
bbRr
br
BbRr
Bbrr
bbRr
bbrr
Fur Color:
Coat Texture:
B: Black
R: Rough
b: White
r: Smooth
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?
BR
Br
bR
br
BR BBRR
BBRr
BbRR
BbRr
Br
BBRr
BBrr
BbRr
Bbrr
bR
BbRR
BbRr
bbRR
bbRr
br
BbRr
Bbrr
bbRr
bbrr
Phenotypic Ratio=
9:3:3:1
Fur Color:
Coat Texture:
B: Black
R: Rough
b: White
r: Smooth
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 (what do you have to draw?) 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.
YyRr X yyrr
YR
Yr
yR
yr
yr
YyRr
Yyrr
yyRr
yyrr
yr
YyRr
Yyrr
yyRr
yyrr
yr
YyRr
Yyrr
yyRr
yyrr
yr
YyRr
Yyrr
yyRr
yyrr
More Practice
• Work on the SpongeBob Dihybrid Cross
worksheet. Turn it in at the end of class.
Mendelian Genetics
Review
Get with a partner.
Walk around the room and answer
the review questions with your
partner.
If you get all the answers correct,
you will get 1 extra point on your
quiz!
1. What is a genotype? The genes (letters)
that determine a phenotype
2. A man is heterozygous for a trait. Will the
dominant or recessive trait be expressed?
Why? Dominant because dominant
covers recessive
3. What does it mean if a woman is
homozygous recessive? Using the letter
“B,” write her genotype. Both of her
genes are recessive (bb)
4. What is a phenotype? The physical
characteristics of an organism
5. Long eyelashes (E) are dominant to short
eyelashes (e). A heterozygous woman is
crossed with a homozygous man. What is
the percent chance their offspring will
have short eyelashes? (Must draw a
Punnett square) 0% (see board)
6. The offspring of two parents has a 100%
chance of being homozygous recessive
for blue eyes. If this is the case, what must
the genotype be for both parents? bb
7. What does homozygous mean? Write an
example. Homozygous means the same
letters are present Ex. BB or bb
8. What does heterozygous mean? Write an example.
That the genes are different Ex. Bb
9. What does it mean if a gene is expressed? The
phenotype for that gene is seen (dominant)
10.What does a dihybrid cross show? Genetic
possibilities for 2 traits
11.What was Mendel’s predicted genotypic ratio for a
typical dihybrid cross? 9:3:3:1
12.Hazel eyes (H) are dominant to green eyes (h). A
homozygous dominant man is crossed with a
homozygous woman. What is the percent chance
their offspring will have green eyes? (Must draw a
Punnett square!) 100% (see board)