Genetic Story Problems
Steps for Genetic Problems
• 1. Identify trait(s)… Dominant (A) and
recessive (a)
• 2. Parents- phenotype and genotype
• 3. Punnett square gametes
• 4. fill out Punnett square
• 5. Offspring Probabilities (%, ratio, : )
– Genotype
– Phenotype
1. Mendel found that the allele for tallness in pea plants (T) is
dominant to the allele for short plants (t). What offspring phenotypes
and genotypes would be expected from the following crosses:
A. Heterozygous and Heterozygous
B. Homozygous dominant and Heterozygous
C. Heterozygous and Homozygous recessive
2. A number of plant species have a recessive allele for albinism (white
leaves). Homozygous albino individuals are unable to make chlorophyll
(green pigments). If a plant heterozygous for this trait is allowed to
self-pollinate and 500 of its seeds are planted:
A. How many of these offspring will
be expected to have the same
genotype as the parent plant:
B. How many seedlings will be
expected to be white?
3. Sniffles, a male mouse with colored coat, was mated with Esmeralda, an
alluring albino. The resulting litter of six young all had colored fur. The next
time around, Esmeralda was mated with Whiskers, who was the same color
as Sniffles. Some of Esmeralda’s next letter were white.
a. What are the probable genotypes
of Sniffles, Whiskers, and
Esmeralda:
b. If a male from the first litter was
mated with a colored fur female
from the second litter, what
phenotype ratios might be
expected among the offspring:
4. In corn plants, the seeds can be purple (A) or yellow (a). If purple
seeds are dominant to yellow corn seeds, predict the genotypes and
phenotypes in the following crosses:
A. two heterozygous purple seeds
B. homozygous purple and yellow
seeds
5. Brown eyes are dominant to blue eyes. Determine the phenotypes and
possible genotypes of the following family members if a brown-eyed man
marries a blue-eyed woman. They have one son with blue eyes and one son
with brown eyes. The man’s father has blue eyes and his mother has brown
eyes. The woman’s parents both have brown eyes.
6. Green pea pods are dominant to yellow pods. Go through the steps
of solving a Punnett square if you are crossing homozygous dominant
plant with a homozygous recessive plant:
7. Black fur is dominant to brown fur in rabbits. If you have two male rabbits
with black fur, show how you could tell if the rabbit was a pure black rabbit or
a hybrid black rabbit. (TEST CROSS)
8. In summer squash, white fruit color is dominant and yellow fruit is
recessive. If a squash plant that is homozygous for white fruit is crossed with
a homozygous yellow fruit plant, what will be the appearance of the first
generation. If two of these plants were crossed, show the genotypes and
phenotypes of the second generation.
F1 generation monohybrid cross
F2 generation monohybrid cross
9. The polled (hornless) trait in cattle is dominant to the horned recessive
trait. A certain polled bull is mated to three cows. Cow A, which is horned,
gives birth to a polled calf. Cow B, also horned, produces a horned calf. Cow C,
which is polled, produces a horned calf. What are the genotypes of the four
parents and three calves.
Bull
Cow A
Calf A
Cow B
Calf B
Cow C
Calf C
10. Suppose that in outer space there exist creatures in which purple
eyes are dominant to yellow eyes. Two purple-eyed creatures mate
and produce six offspring. Four of them have purple eyes and two have
yellow eyes. What are the genotypes and phenotypes of the parents
and offspring.
Co-dominance
1. In horses, the gene for brown coat color is incompletely dominant to the
gene for white coat color. The heterozygous condition results in a tan coat
pattern. Complete the following crosses and determine the genotype and
phenotype ratios of the offspring:
a. Brown horse and a white horse
b. Two tan horses
2. When you cross a pure yellow parrot with a pure blue parrot, all the
offspring are green. Show the Punnett square, the genotype ratios, and the
phenotype ratios if you cross:
a. Two green parrots
b. A green and a yellow parrot
c. A green and a blue parrot
3. In cats, black fur is incompletely dominant to yellow fur. Heterozygous cats
are calico (black, yellow, and white fur). Show the genotypes and phenotypes
if you had:
a. A black and a yellow cat
b.
A calico and a yellow cat
c.
Two calico cats
Dihybrid Crosses
AB
AB
Ab
aB
ab
Ab
aB
ab