Fiducia TogetherDNA and inheritance
How characteristics are passed on, and the vocabulary of a genetic cross.
Please note: This page explains a word or a process in education. It is general information about the system, the law and the guidance in England — not advice about your child, your school or your exam. If a decision is going against you, get advice from one of the free services we link to before you agree to anything or miss a deadline. If you are revising, check your own exam board's specification: boards word things differently and yours is the one that marks you.
What it is
DNA is a double helix carrying genes; a gene codes for a protein. An allele is a version of a gene. Genotype is the alleles present; phenotype is the characteristic expressed. Homozygous means two identical alleles, heterozygous two different ones.
Where it applies
GCSE biology and combined science.
How it works
A dominant allele is expressed when one copy is present; a recessive one needs two. Punnett squares predict the ratios of offspring genotypes.
Inherited conditions such as cystic fibrosis (recessive) and polydactyly (dominant) are the standard examples.
How to use it in your work
Use capital and lower-case letters that are clearly different in handwriting — B and b, not S and s.
Always state the ratio AND what it means: "3:1, so a 75% chance of the dominant phenotype".
Probability applies to each offspring independently. Three affected children do not make the fourth safe.
Where to get help
Your teacher, your board's specification for the depth required at your tier, and Bitesize for the diagrams. The subject song runs the topics in order.
Revise this with a song
These are ours. We write revision songs because a lot of people remember a tune when they cannot remember a page, and because revising out loud in the kitchen beats staring at a highlighter.
- The Biology Song The GCSE biology specification, set to music.
Where to read more
Last reviewed 2026-08-28 by Fiducia Together · Next review due 2027-08-28
Please note: This page explains a word or a process in education. It is general information about the system, the law and the guidance in England — not advice about your child, your school or your exam. If a decision is going against you, get advice from one of the free services we link to before you agree to anything or miss a deadline. If you are revising, check your own exam board's specification: boards word things differently and yours is the one that marks you.