MONOHYBRID CROSSES
PART I.
Complete the following problems, showing the cross (if
not present), set up the Punnett square and determine
both the genotype and phenotype ratios of the offspring.
You will be able to use the key for questions 1-7.
1. a. Rr X RR
TRAIT
KEY
DOMINANT
RECESSIVE
Seed coat shape
Pod color
Height of plant
Round (R)
Green (G)
Tall (T)
Wrinkled (r)
Yellow (g)
Short (t)
b.
c. Genotype Ratio_______________________
d. Phenotype Ratio_______________________
2. a. Gg X GG
b.
c. Genotype Ratio_______________________
d. Phenotype Ratio_______________________
3. a. Tt X Tt
b.
c. Genotype Ratio_______________________
d. Phenotype Ratio_______________________
4. a. Rr X Rr
b.
c. Genotype Ratio_______________________
d. Phenotype Ratio_______________________
5. Cross 2 heterozygous green plants.
a.
b.
c. Genotype Ratio_______________________
d. Phenotype Ratio_______________________
6. Cross a homozygous green plant with a heterozygous green plant.
a.
b.
c. Genotype Ratio______________________
d. Phenotype Ratio______________________
7. Cross a homozygous tall plant with a short plant.
a.
b.
c. Genotype Ratio______________________
d. Phenotype Ratio______________________
8. Cross a white seed plant with a heterozygous brown seed plant. (Hint: Make a key first.)
a.
b.
c. Genotype Ratio______________________
d. Phenotype Ratio_______________________
9. In squash, the allele for white fruits (W) is dominant over the allele for yellow (w). If a white-fruited plant is crossed
with a yellow-fruited plant and all the offspring are yellow, what are the possible genotypes of the parents and the
offspring?
a. Parent’s Genotypes_________________________________
b. Offspring’s Genotypes______________________________
PART II.
A. One of the most remarkable features of human life is that most people are free of genetically related syndromes and
diseases. The genetic mechanism, complex as it is, operates mostly without error. However, when abnormalities or
irregularities occur, the results can be devastating. Tay-Sachs syndrome is a fatal infant condition caused by the
possession of two recessive alleles. This syndrome is especially prevalent in Jews of Central European extraction.
Scientists estimate that one in every 28 Jews originally from this region are heterozygous for the gene that causes TaySachs syndrome.
Complete the two Punnett squares below. In Square A, show the results of a cross between a heterozygous parent (Tt) and
a parent who is homozygous dominant (TT). In square B, show the results of a cross between two heterozygous
individuals. Then answer the questions below.
A.
B.
1. What are the chances that parents in A will have a baby with Tay-Sachs? _______________________
2. What are the chances that parents in B will have a baby with Tay-Sachs? _______________________