- a dominant allele is always expressed in the phenotype, even when there's only one copy of it. dominant alleles are represented by a capital letter. recessive alleles are those with characteristics that only appear in the phenotype if two copies are present. they're shown by lower case letters :

for example, the allele for brown eyes, B, is dominant, so if a person's genotype is Bb or BB they'll have brown eyes. the allele for blue eyes, b, us recessive, so a person will only have blue eyes if their genotype is bb.
the allele for round seed shape, R, is dominant, so if a pea plant's genotype is Rr or RR it will have round seeds. the allele for wrinkled seed shape, r, is recessive, so a pea plant will only have wrinkled seeds if its genotype is rr.

	Created by izzy smith over 7 years ago

basic terms and definitions for the genetics topic, for the aqa biology spec.

genetic terms