Homework: Punnett Squares
Guinea pigs can have either a brown or black coat. The B is the dominant gene for black
coat. The b is the recessive gene for brown coat. Create a cross for the following scenarios.
Include the genotypic and phenotypic ratios.
1. A pure black-coated male is crossed with a pure brown-coated female.
2. A pure black-coated female is crossed with a hybrid black-coated male.
3. A heterozygous black-coated male is crossed with a heterozygous black-coated
female.
4. A homozygous brown-coated male is crossed with a heterozygous black-coated
female.
5. A pure black-coated female is crossed with a pure black-coated male.
6. A homozygous brown-coated female is crossed with a homozygous brown-coated
male.
Make sure to use the correct format. You may answer the questions on a separate sheet
of paper or you may print this out and answer them on the back. Make sure to write out
the questions if you answer on a separate sheet of paper.
Example:
Genotypic Ratio:
BB =
Bb =
Bb =
Phenotypic Ratio:
Black coat color =
Brown coat color =
Punnett Square Practice Problems (cont.)
(Be sure to use the same format that was described on the previous page)
In humans, brown eyes (B) are dominant over blue (b). A brown-eyed man marries a blue-eyed
woman and they have three children, two of whom are brown-eyed and one of whom is blueeyed. Draw the Punnett square that illustrates this marriage.
What is the man’s genotype?
What are the genotypes of the children?
In dogs, there is a hereditary deafness caused by a recessive gene, “d.” A kennel owner has a
male dog that she wants to use for breeding purposes if possible. The dog can hear, so the owner
knows his genotype is either DD or Dd. If the dog’s genotype is Dd, the owner does not wish to
use him for breeding so that the deafness gene will not be passed on. This can be tested by
breeding the dog to a deaf female (dd). Draw the Punnett squares to illustrate these two possible
crosses.
In each case, what percentage/how many of the offspring would be expected to be hearing?
How many of the offspring would be expected to be deaf?
How could you tell the genotype of this male dog?