PRACTICE PUNNETT SQUARE PROBLEMS
COMPLETE DOMINANCE
1. In raccoons, having a thick ring around their eyes is dominant to having a thin ring around the
eyes. If two heterozygous raccoons mate, what percent of the offspring will be heterozygous as well?
2. Sharp teeth are dominant to rounded teeth for certain dinosaurs. A heterozygous dominant
dinosaur is crossed with another dinosaur. Two of their offspring have round teeth and two have
sharp teeth. What is the genotype of the second parent dinosaur?
INCOMPLETE DOMINANCE
3. Fur color shows incomplete dominance and a chocolate Labrador is heterozygous to a golden
Labrador or a black Labrador. What is the genotype of a chocolate Labrador? What percent of the
offspring will be chocolate if you cross a chocolate Labrador with a homozygous dominant
Labrador?
4. Brown eyes and green eyes show incomplete dominance and the heterozygous result is a hazel
eye color. What are the genotypes of the parents if the ratio of eye color is 1 brown eye: 2 hazel
eyes: 1 green eye?
CO-DOMINANCE
5. In butterflies, wing color gene shows co-dominance. When a true-breeding yellow butterfly is
crossed with a true-breeding red butterfly, how many offspring will be heterozygous? What is the
phenotype of the heterozygous offspring?
6. In fish, plain colored fish, such as black and white, are homozygous. The heterozygous fish is striped
with both colors. What type of dominance is displayed in these particular fish? What are all the
possible phenotypes if you cross two heterozygous fish?
SEX-LINKED DOMINANCE
7. Colorblindness is a sex-linked recessive trait. A carrier female mates with a male is who is not
affected by colorblindness. What is the possibility that they will have a girl that is colorblind?
8. Hemophilia is a sex-linked recessive trait. An affected male mates with a carrier female. What is the
possibility that they will have a colorblind son?
PRACTICE PUNNETT SQUARE PROBLEMS ANSWER KEY
COMPLETE DOMINANCE
1. In raccoons, having a thick ring around their eyes is dominant to having a thin ring around the
eyes. If two heterozygous raccoons mate, what percent of the offspring will be heterozygous as well?
Gene: ring size
Allele: thick ring (R); thin ring (r)
Parent Genotypes: Rr x Rr
Answer: 50% of the offspring will be
heterozygous for the ring trait as well.
R
r
R
RR
Rr
r
Rr
rr
2. Sharp teeth are dominant to rounded teeth for certain dinosaurs. A heterozygous dominant
dinosaur is crossed with another dinosaur. Two of their offspring have round teeth and two have
sharp teeth. What is the genotype of the second parent dinosaur?
Gene: teeth shape
Allele: sharp teeth (T); rounded teeth (t)
Parent Genotypes: Tt x tt
Answer: The genotype of the second parent is
homozygous recessive (tt).
t
t
T
Tt
Tt
t
t
t
INCOMPLETE DOMINANCE (the heterozygous traits will be blended)
3. Fur color shows incomplete dominance and a chocolate Labrador is heterozygous to a golden
Labrador or a black Labrador. What is the genotype of a chocolate Labrador? What percent of the
offspring will be chocolate if you cross a chocolate Labrador with a homozygous dominant
Labrador?
Gene: ring size
Allele: thick ring (R); thin ring (r)
Parent Genotypes: Rr x Rr
Answer: 50% of the offspring will be
heterozygous for the ring trait as well.
R
r
R
RR
Rr
r
Rr
rr
4. Brown eyes and green eyes show incomplete dominance and the heterozygous result is a hazel
eye color. What are the genotypes of the parents if the ratio of eye color is 1 brown eye: 2 hazel
eyes: 1 green eye?
Gene: ring size
Allele: thick ring (R); thin ring (r)
Parent Genotypes: Rr x Rr
Answer: 50% of the offspring will be
heterozygous for the ring trait as well.
R
r
R
RR
Rr
r
Rr
rr
**This is a problem where you work backwards, using the offspring ratios to help find parent
genotypes
CO-DOMINANCE (the heterozygous phenotype will feature both traits)
5. In butterflies, wing color gene shows co-dominance. When a true-breeding yellow butterfly is
crossed with a true-breeding red butterfly, how many offspring will be heterozygous? What is the
phenotype of the heterozygous offspring?
Gene: wing color
Allele: yellow (YY), red (yy)
Parent Genotypes: YY x yy
Answer: 100% of the offspring will be
heterozygous and the butterflies wings will be
both red and yellow.
y
y
Y
Yy
Yy
Y
Yy
Yy
6. In fish, plain colored fish, such as black and white, are homozygous. The heterozygous fish is striped
with both colors. What type of dominance is displayed in these particular fish? What are all the
possible phenotypes if you cross two heterozygous fish?
Gene: color
Allele: black (B); white (b)
Parent Genotypes: Bb x Bb
Answer: The possible phenotypes of the
offspring fish are black (25% BB) black and
white (50% Bb) and white (25%bb)
B
b
B
BB
Bb
b
Bb
bb
SEX-LINKED DOMINANCE (remember what an affected, carrier, and normal genotype are)
7. Colorblindness is a sex-linked recessive trait. A carrier female mates with a male is who is not
affected by colorblindness. What is the possibility that they will have a girl that is colorblind?
Gene: colorblindness
Allele: not colorblind (dominant-B); colorblind
(recessive-b)
Parent Genotypes: XBXb x XBY
Answer: 0% chance that the couple will have a
daughter that is colorblind.
XB
Xb
XB
Y
B
B
X X XBY
XBXb XbY
8. Hemophilia is a sex-linked recessive trait. An affected male mates with a carrier female. What is the
possibility that they will have a colorblind son?
Gene: hemophilia
Allele: not hemophiliac (dominant-H);
hemophiliac (recessive-h)
Parent Genotypes: XHXh x XhY
Answer: 50% chance that the couple will have
a son that is colorblind.
XH
Xh
Xh
Y
H
h
X X XHY
XHXh XhY