Edexcel International A Level · Biology
Inheritance
🧬 The Big Idea: Random changes to DNA (mutations) create different versions of genes (alleles). Those alleles get passed from parents to offspring in predictable patterns — and we can now screen for harmful ones before or during pregnancy, though every method comes with real risks and real ethical trade-offs.
Summary — What This Chapter Covers
- Mutations are random changes to DNA base sequences — substitution, insertion, or deletion — and most are harmless, but some change proteins enough to cause disease or (rarely) an advantage.
- Genes and alleles: a gene is a DNA instruction for one polypeptide; an allele is a version of that gene. Individuals can be homozygous (same allele twice) or heterozygous (two different alleles).
- Dominance patterns: dominant alleles always show up in the phenotype; recessive alleles only show up with no dominant allele present; codominant alleles both show up at once.
- Pedigree diagrams trace how a trait moves through a family tree, letting you work out dominant/recessive patterns and individual genotypes.
- Sex linkage: genes on the X chromosome are inherited differently by males and females because males only have one X chromosome.
- Cystic fibrosis is a real-world example of a recessive genetic disorder caused by a faulty CFTR gene, affecting the lungs, gut, and reproductive system.
- Genetic screening comes in three flavours: carrier testing, Preimplantation Genetic Diagnosis (PGD), and prenatal testing (CVS / amniocentesis) — each with benefits and risks.
- Ethical & social issues around screening depend heavily on personal, religious, and cultural viewpoints — exam answers need balance, not opinion.
1. Mutations
What is a mutation, really?
A gene mutation is simply a change in the sequence of bases in a DNA molecule. Think of a gene as a recipe written in a 3-letter code (each triplet of bases codes for one amino acid). A mutation is like a typo in that recipe. Sometimes the typo doesn't change the meaning at all. Sometimes it changes one word. And sometimes it shifts every single word after the typo, turning the whole recipe into nonsense.
Mutations happen spontaneously during DNA replication — they're not something that only happens when something goes wrong from outside; copying over 3 billion bases perfectly, every single time, is just statistically hard. Mutagens (ionising radiation, X-rays, certain chemicals) increase how often mutations occur, but they aren't required for a mutation to happen.
The three point mutations
Substitution, insertion, and deletion are all called point mutations because each one involves a change at a single location in the DNA sequence — but their effects are very different.
Original gene: TAT | AGT | CTT
Tyr Ser Leu
SUBSTITUTION (one base swapped):
Mutated gene: TAT | AGA | CTT
Tyr Arg Leu
→ Only the ONE affected triplet changes. Everything downstream is unaffected.
INSERTION (one base added):
Mutated gene: TAT | AGG | TCT | T..
Tyr Arg Ser ...
→ EVERY triplet after the insertion point shifts. This is a "frameshift".
DELETION (one base removed):
→ Same frameshift problem, just from removing a base instead of adding one.
Key Rule
3. Sex Linkage
Sex-linked genes