Genetics & Inheritance

Interactive Punnett Square Builder — A-Level Biology

Choose a cross type

Key Terminology

Allele: A version of a gene
Homozygous: Two identical alleles (AA or aa)
Heterozygous: Two different alleles (Aa)
Dominant: Expressed when one or two copies present
Recessive: Only expressed when homozygous (aa)
Genotype: The alleles an organism has
Phenotype: The observable characteristic
Codominance: Both alleles expressed equally
Exam tip: Always use a genetic diagram with a key, parental phenotypes, parental genotypes, gametes, Punnett square, and offspring ratios. Marks are awarded for each step — don't skip any, even if the answer seems obvious.