Based on probability and knowledge of Mendelian inheritance, a Punnett square can be used to predict the proportions of different genotypes in the offspring of a given set of parents. Mendel didn’t know about genes, but luckily he studied simple dominant-recessive traits, so he could work backward from phenotypes to determine the pattern of inheritance. More complex patterns of inheritance include codominance, incomplete dominance, multiple alleles, multiple genes, and environmental influences on gene expression.
- CA.9–12.IE.1.a, d, g, j; CA.9–12.LS.2.a, c, d, e, g; CA.9–12.LS.3.a, c
- NSES.9–12.A.1.6; NSES.9–12.A.2.1, 4, 6
- AAAS.9–12.5.B.2; AAAS.9–12.5.F.5; AAAS.9–12.12.B.1
- Define probability.
- Explain how probability is related to inheritance.
- Describe how to use a Punnett square.
- Explain how Mendel interpreted the results of his experiments.
- Describe complex patterns of inheritance.
codominance: relationship between two alleles for the same gene in which both alleles are expressed equally in the phenotype of the heterozygote
incomplete dominance: relationship between the alleles for a gene in which one allele is only partly dominant to the other allele so an intermediate phenotype results
polygenic characteristic: characteristic, or trait, controlled by more than one gene, each of which may have two or more alleles
probability: the likelihood, or chance, than a certain event will occur
Punnett square: chart for determining the expected percentages of different genotypes in the offspring of two parents
Introducing the Lesson
Toss a coin to see if a head or tail turns up, after first asking the class to predict the outcome of the toss. Use the coin toss as a vehicle to introduce the concept of probability. Tell students they will learn how probability is related to heredity when they read this lesson.
Assign the drag-and-drop activity at the URL below. Students will use Punnett squares to determine expected proportions of genotypes and phenotypes of offspring.
Pair ELL and native English speakers, and ask partners to make a cluster diagram for the section “Non-Mendelian Inheritance.” ELL
Have students who need extra challenges solve the case study Those Old Kentucky Blues at the URL below. In this honors activity, they will use pedigrees and other information to determine the inheritance pattern of an interesting genetic condition.
Have students use Mendelian principles to solve the genetics problems at the URL below.
Students commonly think that the expected proportions of genotypes in offspring, as given in a Punnett square, are the actual proportions. Explain that they are only the most likely proportions, and other proportions may occur in any given family.
Reinforce and Review
Copy and distribute the lesson worksheets in the CK-12 Biology Workbook. Ask students to complete the worksheets alone or in pairs as a review of lesson content.
Have students answer the Review Questions listed at the end of the lesson in the FlexBook®.
Points to Consider
Like most of the characteristics of living things, the characteristics Mendel studied in pea plants are controlled by genes. All the cells of an organism contain the same genes, because all organisms begin as a single cell. Most of the genes code for proteins.
- How is the information encoded in a gene translated into a protein? Where does this occur, and what processes are involved?
- If cells have the same genes, how do you think different cells arise in an organism? For example, how did you come to have different skin, bone, and blood cells if all of your cells contain the same genes?