Outcome: Beyond Dominance and Recessiveness

Explain complications to the phenotypic expression of genotype, including mutations.

Mendel’s experiments with pea plants suggested that: (1) two “units” or alleles exist for every gene; (2) alleles maintain their integrity in each generation (no blending); and (3) in the presence of the dominant allele, the recessive allele is hidden and makes no contribution to the phenotype. Therefore, recessive alleles can be “carried” and not expressed by individuals. Such heterozygous individuals are sometimes referred to as “carriers.” Further genetic studies in other plants and animals have shown that much more complexity exists, but that the fundamental principles of Mendelian genetics still hold true. In the sections to follow, we consider some of the extensions of Mendelism. If Mendel had chosen an experimental system that exhibited these genetic complexities, it’s possible that he would not have understood what his results meant.

What You’ll Learn to Do

  • Explain how a trait with incomplete dominance will appear in a population.
  • Explain how a trait with codominant inheritance will appear in a population.
  • Explain how a trait with sex-linkage will appear in a population.
  • Explain how mutli-allele inheritance will impact a trait within in a population.
  • Describe the impacts of penetrance and expressivity on a trait’s expression in a population.

Learning Activities

The learning activities for this section include the following:

  • Reading: Variations of Dominance
  • Reading: Sex-Linked Traits
  • Reading: Multiple Alleles
  • Reading: Penetrance and Expressivity
  • Self Check: Beyond Dominance and Recessiveness