What you’ll learn to do: Explain complications to the phenotypic expression of genotype, including mutations

Mendel’s experiments with pea plants suggested the following:

1. Two “units” or alleles exist for every gene.
2. Alleles maintain their integrity in each generation (no blending).
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.