Practice Problems – Simple Dominance - Answers
1. A pea plant that produced wrinkled seeds (Plant #1) is crossed with a plant that produces round seeds
(Plant #2). All of the 100 offspring produced have round seeds.
A) What trait is being studied? seed shape
B) What are the two possible alleles for this trait? round and wrinkled
C) Which allele is dominant? round (since all of the offspring are round)
D) Which allele is recessive? wrinkled
E) What is the phenotype of Plant #1? wrinkled seeds
F) What is the phenotype of Plant #2? round seeds
G) What is the phenotype of the offspring? round seeds
H) Assume that Plant #1 is homozygous. What is its genotype? rr
R = round
r = wrinkled
I) Assume that Plant #2 is homozygous. What is its genotype? RR
J) What is the genotype of the offspring? Rr
K) Are the offspring homozygous or heterozygous? heterozygous
L) Predict what genotypes and phenotypes you would expect in a cross between 2 of the offspring.
(SHOW YOUR PUNNETT SQUARE.)
R
r
R
r
RR
Rr
Rr
rr
¼ RR
½ Rr
¼ rr
¾ round
¼ wrinkled
M) Show the genotypes for the P cross.
RR x rr
N) Show the genotypes for the F1 generation.
Rr
O) Show the genotypes for the F1 cross.
Rr x Rr
P) Explain who would be the F2 generation.
The ¾ round and ¼ wrinkled offspring shown in the Punnett square above would be the F2
generation.
2. In guinea pigs the black color (B) is dominant to white color (b).
A) What will be the phenotype of a guinea pig with the genotype Bb?
The guinea pig will be black since black is dominant.
B = black
b = white
B) Is the Bb genotype homozygous or heterozygous?
Heterozygous
C) What kind of gametes can a homozygous black guinea pig produce?
All of the gametes will contain B.
D) What kind of gametes can a homozygous white guinea pig produce?
All of the gametes will contain b.
E) What kind of gametes can a heterozygous black guinea pig produce?
Half of the gametes will contain B while the other half will contain b.
F) If you mate 2 black guinea pigs, is it possible to get a white offspring? Explain.
Yes. If the black guinea pig is heterozygous, then each parent could contribute a gamete with b.
B
b
B
BB
Bb
b
Bb
bb
There is a 1 in 4 chance of
having a white offspring.
G) If you mate 2 white guinea pigs, is it possible to get a black offspring? Explain.
No. To be white, a guinea pig must be homozygous. This means each parent only has b alleles to
give. All of the offspring will be white (bb)
b
b
b
bb
bb
b
bb
bb
100% of the offspring will
be white.
H) In a mating between two heterozygous guinea pigs, what fraction of the offspring will have each
phenotype? ¾ will be black and ¼ will be white
b
B
There is a 3 in 4 chance of
having a white offspring.
B
Bb
BB
b
Bb
bb
There is a 1 in 4 chance of
having a white offspring.
3. From a series of crosses between the same two parent guinea pigs offspring were found as follows:
30 long-haired and 10 short-haired.
A) Which allele do you predict is dominant from these results?
Based on the 3:1 ratio, this was most likely a cross between two heterozygous guinea pigs with
long hair being dominant over short hair.
B) What were the most likely genotypes of the parents?
Ll X Ll (where L = long hair and l = short hair)
C) What were the most likely phenotypes of the parents?
They would have long hair.