gen-define-dominant-and-recessive-alleles-and-give-one-short-wor-b80c

Define dominant and recessive alleles and give one short worked example.

# Dominant and Recessive Alleles ## Definitions **Dominant allele**: An allele that expresses its phenotype (observable trait) even when only ONE copy is present. Represented by a capital letter (e.g., **A**). **Recessive allele**: An allele that only expresses its phenotype when TWO copies are present (homozygous). Masked by a dominant allele when paired together. Represented by a lowercase letter (e.g., **a**). ## Worked Example: Flower Color in Pea Plants - **B** = purple flowers (dominant) - **b** = white flowers (recessive) **Cross**: Bb × Bb (two purple-flowered plants) | | B | b | |---|---|---| | **B** | BB | Bb | | **b** | Bb | bb | **Results**: - **BB** (purple) - 25% - **Bb** (purple) - 50% - **bb** (white) - 25% **Phenotype ratio**: 3 purple : 1 white **Key point**: Both BB and Bb plants have purple flowers because B is dominant. Only bb plants have white flowers because two copies of the recessive allele are needed for the white phenotype to appear.