Name ________________________________________
Solving Problems in Genetics.Answer Key
Date ______
1. For coat color, B represents the allele for brown color. And b represents the
allele for white color. Draw a Punnett square to show the cross Bb x Bb.
B
B
b
BB
Bb
Bb
bb
b
b
a. What percentage of the offspring have the same phenotype as the
parents?
75% of offspring are brown
b. What percentage of the offspring have the same genotype as the
parents?
Parents’ genotype: Bb – heterozygous or hybrid
50% of the offspring are also heterozygous or hybrids
c. What percentage of the offspring may have a white coat?
1 in 4 or ¼ or 25% may have a white coat
2. For stem height, T is the allele for tall and t is the allele for short. What is
the phenotype of the offspring if you crossed two parents who were
homozygous recessive for height?
Homozygous recessive = tt
If you cross two parents that are tt (tt x tt) the phenotype of all the
offspring will be short.
2. If Y represents the allele for yellow seeds and y represents the allele for
green seeds, what would be the hybrid form (genotype)?
Yy
3. A short-haired cat (H?) is crossed with a longhaired cat (hh).
a. In the F1 generation there is a cat born with long hair. Is the shorthaired cat in the P generation a hybrid or a purebred?
H
h
h
? = Hh
Hh
?h
Hh
?h
The short-haired cat in the P (parent generation) is a hybrid - Hh
b. If the short-haired cat in the P generation were purebred, what
would you expect the offspring to look like?
H
h
h
H
Hh
Hh
Hh
Hh
All the offspring are short-haired (Hh)
5. A family has four daughters.
a. What is the probability that a fifth child will be a girl?
50/50
b. Does the fact that there are already four daughters in the family increase
the probability of having another girl? Explain.
All events in probability occur independently of previous events;
therefore they still have a 50/50 chance of having a boy or girl.
6. A widow’s peak allele is dominant (W) and the straight hairline allele is
recessive (w).
How could parents with widow’s peaks have a child without the trait?
If both parents are hybrids (Ww) they can have a child a child without the
Trait (ww).
W
W
w
w
WW
Ww
Ww
ww
h
7. A white flower (pp) is crossed with a purple flower (Pp).
a. Draw a Punnett square to illustrate the cross.
P
p
p
Pp
pp
p
Pp
pp
b. What would the possible genotypes of the offspring be?
Pp = heterozygous or hybrid
pp = homozygous recessive
c. For the above cross, what would the phenotypes be?
Genotype Pp = purple flower
Genotype pp = white flower
8. Examine the Punnett square below, and then answer the following questions:
?
?
T
TT
TT
t
Tt
Tt
a. What is the unknown genotype?
TT
b. If T represents the allele for tall pea plants, and t represents the allele
for short pea plants, what is the phenotype of each parent and of the
offspring?
All parents and all offspring are tall.
c. If each of the offspring were allowed to self-fertilize, what are the
possible genotypes in the next generation?
T
T
TT
T
TT
T
T
T
t
TT
Tt
Tt
tt
?
T
TT
TT
t
d. What is the probability of each genotype in question c?
TT x TT
Tt X Tt
TT = 100%
TT = 25 %
Tt = 50%
tt =25%