Monohybrid Genetics Problems
PURPOSE: To predict and compare the genetic variations that will result from a genetic cross
PUNNETT SQUARE REVIEW
Label the lines around the Punnett square with the following terms: gametes from parent 1, gametes from
parent 2, and offspring.
SETTING UP CROSSES REVIEW:
1. What is used to represent the genes of an organism?
2. How is the letter used determined?
3. What kind of letter is used for dominant traits?
4. What kind of letter is used for recessive traits?
PRACTICE PROBLEMS:
1. In a certain plant, the gene for red flowers (R) is dominant to the gene for white flowers (r)
a) What is the genotype of a heterozygous plant?
b) What is the genotype of a homozygous red flowered plant?
c) What is the genotype of a white flowered plant?
d) What is the phenotype of a plant with two dominant genes?
e) What is the phenotype of a plant with 1 dominant and 1 recessive gene?
2. Homozygous Red x Homozygous White
a) genotypic ratio:
b) phenotypic ratio:
c) What is the probability of having heterozygote offspring?
d) What is the probability of having offspring expressing the dominant trait?
3. Heterozygote x Heterozygote
a) genotypic ratio:
b) phenotypic ratio:
c) What is the probability of having heterozygous offspring?
d) What is the probability of having offspring expressing the dominant trait?
e) What is the probability of having offspring expressing the recessive trait?
4. Dragons have either smooth or wrinkled noses. Wrinkled is dominant over smooth. Cross a smoothnosed female dragon and a heterozygous wrinkle-nosed male.
a) genotypic ratio:
b) phenotypic ratio:
c) What is the probability of having smooth-nosed offspring?
d) What is the probability of having two smooth-nosed offspring?
5. Mendel found that the allele for tallness in pea plants (T) is dominant to the allele for shortness (t).

Heterozygous plant that self fertilizes
a) genotypic ratio:
b) phenotypic ratio:
c) probability of having a homozygous tall plant:
d) When a plant fertilizes itself, is there still genetic variation between the offspring and the parent?

Homozygous tall x heterozygous
a) genotypic ratio:
b) phenotypic ratio:
c) probability of having a heterozygote plant:

Heterozygous and homozygous short
a) genotypic ratio:
b) phenotypic ratio:
c) Probability of having a homozygous short plant:

Homozygous tall and homozygous short
a) genotypic ratio:
b) phenotypic ratio:
c) probability of having two homozygous short plants in a row:
6. Two fruit flies with normal wings are crossed. Among their offspring are 89 with normal wings and 35
with dumpy wings.
a. What are the genotypes of the parents?
b. Which trait is dominant? Which trait is recessive? Explain how you know.
7. Sniffles, a male mouse with a brown coat, was mated with Esmeralda, an albino (white) mouse. The
resulting litter of six young all had brown fur. The next time around, Esmeralda was mated to
Whiskers, a male the same color as Sniffles. Some of Esmeralda’s next litter was white.
What are the probable genotypes of…
a. Sniffles?
b. Whiskers?
c. Esmeralda?
8. A number of plant species have a recessive allele for albinism; homozygous albino individuals are
unable to synthesize chlorophyll. If a heterozygous tobacco plant is allowed to self pollinate and 500
of its seeds germinate…
a. How many of these offspring will be expected to have the same genotype as the parent plant?
b. How many seedlings will be expected to be albino?
A dominant phenotype could have two possible genotypes: Homozygous dominant (DD) or Heterozygous
(Dd). To determine the genotype of an organism showing the dominant phenotype, a test cross should be
performed. In a test cross, the unknown dominant phenotype is crossed with a known recessive
phenotype.
Pretend you own a plaid tribble. Plaid (P) is dominant to striped (p)in tribbles. You need to determine
whether your plaid tribble is PP or Pp.
Test cross #1: Homozygous plaid x striped
Genotypic ratio:
Phenotypic ratio:
Probability of heterozygote offspring:
Probability of striped offspring:
Test cross #2: Heterozygous plaid x striped
Genotypic ratio:
Phenotypic ratio:
Probability of heterozygote offspring:
Probability of striped offspring:
Conclusion:
If the recessive trait shows in the offspring of the test cross above,
the unknown plaid parent’s genotype is: