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Dihybrid Genetic Problem Set:
1. In summer squash, white fruit is due to a dominant gene and colored
fruit is due to its recessive allele. Disc shaped fruit is dominant over
sphere shaped fruit.
a. Show all the possible genotypes of a white/disc shaped squash.
b. Cross a heterozygous white/disc shaped squash with a heterozygous
white/disc shaped squash.
c. How many phenotypes result from this cross? What are they? In what
frequency? In what ratios to each other?
2. In cocker spaniels, black is due to a dominant gene (B), red to its
recessive allele (b). Solid color is dependent on a dominant gene (S)
and white spotting on its recessive allele (s). A red male was mated to
a black and white female. They had five puppies: one black, one red,
two red and white, and one black and white. What were the expected
genotypes of the two parents? Show the cross.
3. In cattle the allelic genes red (R) and white (r) show co-dominance, the
heterozygous animal having roan coat color. Curly coat (s) is
recessive to straight coat (S).
a. Show a cross between a curly red bull and a pure-breeding straight,
white cow. What would be the expected offspring phenotype?
b. Genotype?
c. Show the Punnett square cross between a bull from part b and a
homozygous curly-haired, roan cow.
4. In poultry, black color is due to a dominant gene (E) and red to its
recessive allele (e). Crested head is due to a dominant gene (H) and
plain head to its recessive allele (h). A red crested male is mated to a
black plain female. They produce many offspring, half of which are
black crested and half of which are red crested. A. What are the
genotypes of the parents? Show the cross.
b. Two black crested birds are mated. They produce 13 offspring
as follows: 7 black crested; 3 red crested;2 black plain; 1 red plain.
What were the genotypes of the parents? Show the cross.
5. In certain bacteria, an oval shape is dominant over round and thick cell
walls are dominant over thin. Cross a heterozygous oval, thick cell
walled bacteria with a round, thin cell walled bacteria (Create a Punnett
square). Describe the phenotype and frequency of the offspring.
6. Bonus:
In guinea pigs, black coat color is dominant over white, short hair is
dominant over long, and rough coat is dominant over smooth. Show
the Punnett Square for a cross between a heterozygous black, shorthaired, rough coat guinea pig and a homozygous white, long-haired,
heterozygous rough coat guinea pig. What do the offspring look like?