Dominant and recessive traits
ScienceDefinition
A dominant allele expresses its phenotype even when only one copy is present (heterozygous), while a recessive allele only shows its phenotype when two copies are present (homozygous recessive). This concept was first described by Gregor Mendel.
Examples
- Brown eye color (B) is dominant over blue (b), so Bb individuals have brown eyes
- Huntington's disease is caused by a dominant allele—only one copy is needed to develop the condition
- Cystic fibrosis is recessive—a person must inherit two copies of the recessive allele (ff) to be affected
Key Fact
Dominant = expressed in Bb (heterozygous); Recessive = only expressed in bb (homozygous recessive).
Study This Concept
Practice dominant and recessive traits with free review games in these units: