Lesson 05-06
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 be represented by 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 or Aa x aa
A A A a
a Aa Aa a
a
Aa Aa
a
Cross a heterozygous black mouse with another heterozygous black mouse. Use letter “A.”
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Genetics Problems, cont.
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.) Use the letter “T.”
2. Cross a black mouse with a white mouse. Although you don’t know the genotype of the black mouse, there are white offspring. Use the letter “B.”
3. Cross a person who is homozygous for unattached earlobes with a person who has attached earlobes. (The allele for unattached earlobes is dominant.) Use the letter “E.”
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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 homozygous 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.
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Genetics Problems, cont.
Example: Cross a sheep with white wool with a sheep that has darker-colored wool. We will assume that white wool is dominant over darker-colored 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 that all of the offspring would have white wool, and none of the offspring would have darker-colored wool.
A A a Aa Aa a Aa Aa
Problem: Cross a heterozygous sheep with another heterozygous sheep, and give the genotypic and phenotypic ratios. Use the letter “A.”
Genotypic ratio: ______________
Phenotypic ratio: _____________
© 2013 BJU Press. All rights reserved.

Genetics Problems, cont.
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. Use the letter “A.”
Genotypic ratio: ________ Phenotypic ratio: _________
5. Cross a heterozygous tall pea plant with another heterozygous tall pea plant. Use the letter “T.”
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. Use the letter “B.”
Genotypic ratio: ________ Phenotypic ratio: _________
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© 2013 BJU Press. All rights reserved.