GENETICS Practice
Ms. Golphin
Science
What is a Punnett Square?
• Punnett square: a
diagram that is used to
predict the genotypes
and phenotypes of an
offspring. Each box is
worth 25%, the total of
a punnett square is
100%
How to complete a Punnett square
Step 1: The genes on the top are
brought down to the boxes below it.
T
t
T
T
T
T
T
t
t
t
How to complete a Punnett square
Step 2: The genes are pushed into
the boxes to the right of them.
T
t
T
T
T
T
T
T
Try These Punnett Squares on your
worksheet
3
1
2
HOMOZYGOUS
• Definition: 2 genes that are
the same. It could be 2
dominant genes or 2
recessive genes
• Example. ff or FF
• Example. 2 blue eyed genes
• Example. 2 brown genes
HETEROZYGOUS
• Definition: 2 genes that
are different. There is 1
dominant gene and 1
recessive gene.
• Example. Rr or tT
• Example. 1 blue gene
and 1 brown gene
G
g
G GG Gg
g Gg gg
1. The parents in this cross
are _____________?
Homozygous OR Heterozygous
2. If G is dominant for green pods and g is recessive
for yellow pods, what percentage of the offspring will
have green pods? _______%
GENOTYPE
• Definition: The 2
letters that stand for
a gene. You get
ONE from each
parent.
• Example. eE, QQ,
Ll, rr
B b
B
b
3. The genotype of the
offspring in the blue box
is _______?
4. The offspring in the blue box is
homozygous OR
heterozygous
PHENOTYPE
• Definition: what
the gene actually
looks like.
• Example. Red
hair, Dark Skin,
Tongue roll
Label the following as genotype or
phenotype.
•
•
•
•
•
•
•
•
A- Small nose
B- Long toes
C- HH
D- 5 feet tall
E- gG
F- Pp
G- Green eyes
H- Qq
5. In guinea pigs black fur (B) is dominant over brown
fur (b). The Punnett square for a cross between a
HETEROZYGOUS black guinea pig and a
HOMOZYGOUS brown guinea pig would look like
B b
B B
B b
B b
B
b
b
B
b
b
b
B
A
B
C
Answer_______?__________
D
R
r
r
Rr
Rr
r
rr
rr
Red throats (R) are
dominant over white (r)
throats in Goonie birds.
Make a cross between a
PURE RECESSIVE
and a
HETEROZYGOUS
Goonie bird.
6. What are the possible phenotypes of
their offspring?
(% and color)
B
b
B
BB
Bb
b
Bb
bb
7.
Black eyes (B) are
dominant over red eyes
(b) in rats.
Make a cross between
two HETEROZYGOUS
rats.
What are the possible phenotypes of
their offspring?
(% and color)
8. The parents in this
cross are _____________
Homozygous
Heterozygous
9. If W is dominant for long wings and w is recessive
for short wings, what percentage of these offspring
will have short wings? _______%
10. What kind of cross is this?
-Pure dominant X pure recessive?
-Heterozygous X heterozygous?
-Heterozygous X homozygous?
-Hybrid recessive X hybrid pure?
Another name for heterozygous is
__Hybrid_____________
A a
A
a
11. The genotype of the
offspring in the blue box
is _______
homozygous
heterozygous
How many of the offspring from this cross will show
the dominant characteristic?
¾ (75%) – AA and Aa
G g
G
g
G is dominant for green pods.
g is recessive for yellow pods.
GG Gg
gg
What is the genotype of the
offspring in the shaded box?
What is the phenotype of the
offspring in the shaded box?
Gg
Green pods
P
p
p
P
12. Purple (P)is the dominant
color for monsters. Yellow is
recessive (p).
Make a cross between a
HOMOZYGOUS purple and
a yellow monster.
What will the offspring be like?
(% and color)
What genotype would a PURPLE dad have to be to
make a yellow monster baby with a Purple (Pp)
mom monster?
P
?
?
p
pp
Pp
Dad needs to have:
a little p to give one to the baby
a big P to be purple himself