Chapter 8 Summary

The topics covered in this chapter can be summarized as follows:

• Phenotypes depends on the alleles that are present, their dominance relationships, and sometimes also interactions with the environment and other factors.
• The alleles of different loci are inherited independently of each other, unless they are genetically linked.
• Many important traits show continuous, rather than discrete variation. These are called quantitative traits.
• Many quantitative traits are influenced by a combination of environment and genetics.
• The expected phenotypic ratio of a dihybrid cross is 9:3:3:1, except in cases of linkage or gene interactions that modify this ratio.
• Modified ratios from 9:3:3:1 are seen in the case of recessive and dominant epistasis, duplicate genes, and complementary gene action. This usually indicates that the two genes interact within the same biological pathway.
• There are other factors that alter the expected Mendelian ratios.