Module 6 Lesson 2 - 8

Lesson Summary

During this lesson, you were to focus on the following essential questions:

• What are the simple principles of single trait inheritance?

• How can genotype be determined from phenotype?

A key part to understanding inheritance is to be able to determine the traits present in past generations and predict which are likely to present themselves again in future generations. By learning to use Punnett squares and test crosses, you can work with probabilities and outcomes. You could counsel a young couple on the probability of having a child with cystic fibrosis if you knew their genotypes. (Cystic fibrosis is a single gene trait.) You could also conduct tests on plants to see if a strain is true breeding or not, and thus whether it is worth keeping for reproduction. With these skills, you can now begin to understand family histories.

Mendel worked with seven contrasting characteristics in peas. Each characteristic was either dominant or recessive; there was no blending in his work. You have learned that dominant traits are expressed in either homozygous dominant or heterozygous plants, but recessive traits can be seen only in homozygous recessive plants. You have learned how to predict the genotypes and phenotypes of offspring using Punnett squares.

Because dominant alleles can hide recessive alleles, breeders and farmers are most interested in working with homozygous or true breeding individuals. This ensures the continuation of desired traits. Because genotypes cannot be seen but only inferred from probabilities, using a test cross is an excellent way to determine genotype.