Answer: One of Gregor Mendel's greatest contributions to the study of heredity was the concept of dominance.
Mendel observed that a heterozygote offspring can show the same phenotype as the parent homozygote, so he concluded that there were some traits that dominated over other inherited traits.
However, the relationship of genotype to phenotype is rarely as simple as the dominant and recessive patterns described by Mendel.
As the study of inheritance expanded beyond the seven traits Mendel initially examined and also included organisms other than pea plants, biologists began to notice a variety of relationships between alleles that code for the same trait.
These allelic interactions were not exclusively recessive or dominant, and they greatly enriched our understanding of how genotype leads to phenotype.