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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
The key difference
A dominant allele is expressed in both the homozygous (AA) and heterozygous (Aa) genotype: only one copy is needed for the trait to appear. A recessive allele is only expressed in the homozygous recessive (aa) genotype: two copies are needed.
Understanding dominant and recessive alleles is the foundation of genetic crosses at IGCSE. You must be able to use genetic diagrams, Punnett squares, and the correct terminology to predict offspring ratios. This guide covers what examiners expect you to know.
Feature
Dominant Allele
Recessive Allele
Symbol
Capital letter (e.g. A, B, T)
Lower case letter (e.g. A, b, t)
Expression
Expressed in both homozygous (AA) and heterozygous (Aa) genotypes
Only expressed in the homozygous recessive (aa) genotype
Copies needed for expression
One copy is sufficient
Two copies needed (one from each parent)
Masked by
Not masked: always expressed when present
Masked by the dominant allele in heterozygous individuals
Carriers
Cannot be a "carrier" for a dominant trait
Heterozygous individuals (Aa) are carriers: they carry the recessive allele but do not show the trait
Example
Brown eye colour (B) is dominant over blue
Blue eye colour (b) is recessive: only expressed as bb
Key Genetic Terminology
Gene: a length of DNA that codes for a specific protein
Allele: a version of a gene (e.g. Brown eye allele vs blue eye allele)
Genotype: the combination of alleles an organism has (e.g. Aa, BB, tt)
Phenotype: the observable characteristic that results from the genotype (e.g. Brown eyes)
Homozygous: two identical alleles (AA or aa)
Heterozygous: two different alleles (Aa)
Why Two Carrier Parents Can Have an Affected Child
If both parents are heterozygous (carriers) for a recessive condition: genotype Aa: they do not show the condition themselves (the dominant allele masks the recessive one). However, using a Punnett square:
A
a
A
AA
Aa
a
Aa
aa
There is a 1 in 4 (25%) chance that each child will be homozygous recessive (aa) and show the recessive trait. This is why genetic conditions like cystic fibrosis and sickle cell anaemia can appear in families with no previous history of the condition: both parents were unknowing carriers.
Try a comparison question (3 marks)
In pea plants, the allele for tall stems (T) is dominant over the allele for short stems (t). Two heterozygous tall plants are crossed. What is the expected ratio of tall to short offspring? Show your working. [3]
Show the answer and marking guidance
Parental genotypes: Tt × Tt
Punnett square or genetic diagram showing gametes T and t from each parent → offspring TT, Tt, Tt, tt