Biology
Cole
Name___________________________________
Period ____________
Genetics In-Class Worksheet #2
Due Ultimately on Wednesday, March 13th, can turn in any time before
On all problems, SHOW YOUR WORK. Show all crosses, using Punnett Squares. Use P to
designate your parents, F1 for your first generation of offspring, and F2 if you have a second
generation of offspring. Report all of the expected phenotypes of offspring using fractions, ratios or
percents.
Dihybrid Crosses
1. A guinea pig has a gene for straight hair (H) versus wavy hair (h), as well as a gene for rough coat
(R) versus smooth (r). Show a cross between a guinea pig that is homozygous dominant for both traits
with one that is heterozygous for both traits. You may abbreviate your Punnett Square if possible.
2. Using the same traits as the above problem, cross a guinea pig who is heterozygous for both traits
with a pig who is homozygous recessive for both traits. You may abbreviate your Punnett Square if
possible.
3. In pea plants, round seeds (R) are dominant over wrinkled (r) seeds. Also, tall plants (T) are
dominant to short (t) plants. Cross a plant that is homozygous dominant for both traits with a plant that
is homozygous recessive for both traits. For a second generation, cross two of the F1 offspring with
each other. Show 2 Punnett Squares. You may abbreviate your Punnett Square if possible.
Incomplete dominance
4. In a flower called Snapdragons, there are three possible phenotypes: red, white, or pink. Pink
flowers result from the heterozygous genotype. Show a cross between two pink snapdragons. Use R
for the red flower allele and W for the white flower allele.
Codominance
5. In some chickens, there is a gene for feather color that exhibits codominance. If a chicken has one
black feather allele and one white feather allele, this heterozygous chicken will be speckled black and
white. For an F1 cross, show a cross between a black chicken and a white chicken. For the F2
generation, cross one of the F1 offspring back with its black parent. Use B for the black feather allele,
and W for the white feather allele.
CHALLENGE DIHYBRID CROSSES (tough- you can do it!!!)
1. Tay Sachs Disease is a recessive disorder which kills any affected individuals by the age of 5. The
disease does not allow lipids to be broken down in the brain properly. Use L for normal lipid
degradation, and l for Tay Sachs.
Remember that the gene for hanging earlobes (H) is dominant over the gene for attached
earlobes (h).
Show a dihybrid cross between one parent who is a carrier for Tay Sachs disease and is
heterozygous for hanging earlobes and one parent who is also a carrier for Tay Sachs disease and has
attached earlobes. You may abbreviate your Punnett Square if possible.
2. Remember that Huntington’s Disease is a dominant disorder affecting the brain.
Remember that Achondroplasia is lethal (resulting in death) when homozygous dominant, and
results in dwarfism in a heterozygous person.
A man who is a dwarf due to Achondroplasia AND is homozygous for Huntington’s Disease
marries a woman who is also a dwarf due to Achondroplasia but she is totally normal for Huntington’s
(not a carrier). Use a dihybrid cross to predict their possible children. Remember that the disorder can
be lethal when you are counting their offspring.