Mendelian Genetics:
Part Two: Punnett Squares
Answer the following on a separate piece of paper.
1. A plant with red flowers is crossed with a plant with white flowers. The
offspring are all pink.
a. Show the cross on a Punnett square.
b. Explain, using the terms phenotype and genotype, why all the
offspring are pink.
2. You now breed two flowers from the F1 generation in question 1.
a. Show the cross on a Punnett square.
b. Explain, using the terms phenotype and genotype, what colors the
offspring will be, and the statistical probability of getting offspring
of each color.
3. Using a Punnett square show the cross between a true breeding pea plant
having green pods (dominant) with a true breeding plant with yellow pods
(recessive). Use G to represent the dominant alleles and d to represent the
recessive.
a. Show the parental cross on a Punnett Square.
b. What are the genotypes of the F1 generation?
c. What are the phenotypes of the F1 generation?
4. Now cross two of the F1 generation from question #3 and show the
phenotypic and genotypic ratios of the offspring.
a. Show the F1 cross on a Punnett Square.
b. What are the genotypes of the F2 generation?
c. What are the phenotypes of the F2 generation?
5. Genetics of Sickle cell anemia
For some genes, neither allele is completely dominant, so a heterozygous
individual differs from either type of homozygous individual.
The gene for hemoglobin is an example. One allele codes for normal hemoglobin,
while another allele codes for altered hemoglobin, called sickle cell hemoglobin.
When a person is homozygous for this sickle cell allele, this causes a serious
disease called sickle cell anemia. The sickle cell hemoglobin tends to cause the
red blood cells to assume a sickle shape, in contrast to the normal disk-shaped
red blood cell. What problems might be caused by the sickle-shaped red blood
cells?
A person who is heterozygous for the sickle cell allele often has very few or no
symptoms of sickle cell anemia. However, people who are heterozygous for the
sickle cell allele are more resistant to malaria, an infection of the red blood cells
that is transmitted by mosquitoes in many tropical countries. Thus, in areas
where malaria is widespread, people who are heterozygous for the sickle cell
allele are less likely to become ill and die. Because of this advantage, the sickle
cell allele became relatively common in regions like West Africa where malaria is
common. Since African-Americans are descended from populations in which the
sickle cell allele was relatively common, African-Americans have relatively high
rates of the sickle cell allele (approximately 8% are heterozygous for this allele
and 0.16% are homozygous).
a. Define homozygous.
b. Define heterozygous.
c. Suppose that a person who is heterozygous for the sickle cell allele
(SK/sk) marries a person who is also heterozygous for this allele
(SK/sk). Draw a Punnett Square to show the expected genetic
makeup of their children, and then answer the following questions.
i. What fraction of their children will suffer from sickle cell
anemia?
ii. What fraction of their children will inherit the sickle cell
allele, but will not suffer from sickle cell anemia? (These
children will be resistant to malaria.)
6. Dihybrid cross
A pea plant is heterozygous for both seed shape and seed color. S is the allele for
the dominant, spherical shape characteristic; s is the allele for the recessive,
dented shape characteristic. Y is the allele for the dominant, yellow color
characteristic; y is the allele for the recessive, green color characteristic.
a. What is the genotype of the parent plant?
b. What are the possible genotypes of the plant’s gametes (egg and sperm)?
c. Draw a Punnett square of a dihybrid cross between these parent gametes.
Remember, this Punnett square looks significantly different from one
drawn for a monohybrid cross.
d. What are the possible genotypes of the offspring?
e. What is the ratio of the genotypes?