Genetics Problems Worksheet
Simple Monohybrid Crosses
A simple monohybrid cross deals with only one set of characteristics
caused by one pair of genes.
Example: Cross a white mouse with a black mouse. We will assume
that fur color is determined by one pair of genes. Therefore, we will
designate a letter of the alphabet to represent this trait. Homozygous
black will be represented by “AA,” and homozygous white (the only
way we would have white) will use the letters “aa.” A heterozygous
black mouse would be represented with “Aa.”
Problem: The above cross could be done two ways since we do not
know the genotype of the black mouse.
AA
X
aa
A
A
a
Aa
Aa
a
Aa
Aa
or
Aa
X
aa
A
a
a
Aa
aa
a
Aa
aa
Cross a heterozygous black mouse with another
heterozygous black mouse. Use letter “A.”
Complete the following practice problems:
1. Cross two people who can taste PTC. One of the children cannot
taste PTC. (The ability to taste PTC is dominant.)
2. Cross a black mouse with a white mouse. Although you don’t know
the genotype of the black mouse, there are white offspring.
BIOLOGY Genetics Problems Worksheet
© 2009 BJU Press. All rights reserved.
3. Cross a person who is homozygous for unattached earlobes with a
person who has attached earlobes. (Unattached earlobes is
dominant.)
Genetic Ratios in Simple Monohybrid Crosses
The probability of offspring having a particular genotype is expressed
in a genotypic ratio. In a simple monohybrid cross, a genotypic ratio
gives the probability of offspring expressing the three possible
genotypes. Always list homozygous dominant first, then heterozygous,
and recessive individuals last.
The probability of offspring expressing a physical characteristic is
expressed in a phenotypic ratio. In a simple monohybrid cross, a
phenotypic ratio gives the probability of offspring expressing either
the dominant or the recessive physical characteristic. Always list
dominant first and recessive last.
Example: Cross a sheep with white wool with a sheep that has darkercolored wool. We will assume that white wool is dominant over darkercolored wool and that the sheep with white wool is homozygous
dominant. Use the letter “A.”
From this cross, we see that all of the offspring
are heterozygous dominant. Therefore, the
genotypic ratio is 0:4:0.
Since all of the offspring have white wool, the
phenotypic ratio is 4:0, which means 4 sheep
have white wool, and none of the sheep have
darker-colored wool.
BIOLOGY Genetics Problems Worksheet
© 2009 BJU Press. All rights reserved.
A
A
a
Aa
Aa
a
Aa
Aa
Cross a heterozygous sheep with another heterozygous sheep, and
give the genotypic and phenotypic ratios.
Genotypic ratio:
Phenotypic ratio:
Do the following practice crosses using the Punnett squares below, and
give both the genotypic and phenotypic ratios.
4. Cross a heterozygous sheep with a sheep with dark-colored wool.
Genotypic ratio:
Phenotypic ratio:
5. Cross a heterozygous tall pea plant with another heterozygous tall
pea plant.
Genotypic ratio:
Phenotypic ratio:
6. In humans, a cleft chin is dominant over a smooth chin. Bob, who
has a cleft chin, marries Sue, who has a smooth chin. One of their
three children has a smooth chin. Using the Punnett square, show
the cross, and give the genotypic and phenotypic ratios.
Genotypic ratio:
BIOLOGY Genetics Problems Worksheet
© 2009 BJU Press. All rights reserved.
Phenotypic ratio: