Genetic Practice Problems w/ Answers
1) A sexually reproducing animal has two unlinked genes, one for head shape (H) and one for tail
length (T). Its genotype in HhTt. What genotypes are possible in the gametes of this organism?
HT, Ht, hT, hT
2) Black fur in mice (B) is dominant to brown fur (b). Short tails (T) are dominant to long tails (t).
What fraction of the progeny of the cross BbTt x BBtt will have black fur and long tails? (Hint:
determine the genotypes of the gametes first, then do the actual cross)
BbTt
BT
Bt
bT
bt
x BBtt
Bt
= B_tt
1/2
(4 different gametes x 1 gamete type)
3) In certain plants, tall is dominant to short. If a heterozygous plant is crossed with a homozygous
tall plant, what is the probability that the offspring will be short?
Tt
X TT = tt
0%
4) Plants with the dominant allele (D) have dark green leaves, and plants with the homozygous
recessive dd genotype have light green leaves. A true-breeding dark-leaved plant is crossed with
a light-leaved one. A) Show this cross. The F1 offspring is allowed to self-pollinate. B) Show this
cross.
A) DD x dd
All Dd
B) Dd x Dd
DD=1, Dd=2, dd=1
5) A couple has 3 children, all of whom have brown eyes and blond hair. Both parents are
homozygous for brown eyes (BB), but one is a blond (rr) and the other is a redhead (Rr). What is
the probability that their next child will be a brown-eyed redhead?
BBRr x BBrr = B_rr
BR
Br
Br
½ or 50%
(2 different gametes x 1 gamete type)
6) Tallness (T) is a dominant to dwarfness (t), while red (R) flower color is dominant to white (r). The
heterozygous condition results in pink (Rr) flower color. A dwarf red snapdragon is crossed with a
plant homozygous for tallness and white flowers. What are the genotype(s) and phenotype(s) of
the F1 individual?
ttRR x TTrr
tr
Tr
(only 1 gamete type from each)
genotype: all TtRr
phenotype: all tall, pink
7) All the offspring of a cross between a black-eyed mendelien and an orange-eyed mendelien have
black eyes. What is the expected phenotypic ratio of a cross between two orange-eyed
mendeliens?
Black (B) is dominant, orange (b) is recessive
bb x bb = all bb
phenotypic ratio is: 1
8) One gene has alleles A and a, a second gene has alleles B and b, and a third gene has alleles C
and c. A) List the possible gametes that can be formed from the following genotypes:
1. Aa
2. BbCc
3. AAcc
1: A and a
2: BC, Bc, bC, bc
3: Ac
9)
For the same genes, what are the predicted genotype and phenotype ratios of the following
genetic crosses?
1. Aa x aa
2. BB x bb
3. AABb x aabb
(2 different gametes x 1 gamete type)
4. BbCc x BbCC
(4 different gametes x 2 different gametes)
1: 1 to 1 (Aa : aa)
2: 1 (all Bb)
3: 1:1 (AaBb : Aabb)
4: 1:1:1:1:1:1:1:1
(BBCC : BBCc : BbCC : BbCc : BbCC : BbCc : bbCC : bbcc)
10) If blue flower color (B) is dominant to white flower color (b), what are the genotypes of the parents
in the following genetic cross: blue flower x white flower yields only blue-flowered offspring?
BB x bb
11) The fruit pods of peas can be yellow or green. In one of his experiments, Mendel crossed plants
that were homozygous for the allele for yellow fruit pods with plants that were homozygous for the
allele for green fruit pods. All fruit pods in the F1 generation were green. Which allele is dominant,
the one for yellow or the one for green? Show why by doing a cross for each.
Green is dominant
YY x yy give all Yy (yellow)
GG x gg give all Gg (green)