GCSE Biology Inheritance, Variation & Evolution Notes
DNA carries the instructions to build an organism. Meiosis produces gametes with genetic variation, and Punnett squares predict inheritance patterns.
Key concepts
- DNA
- Double-helix polymer of nucleotides found in the nucleus. Genes are sections of DNA that code for a specific protein.
- Allele
- Different versions of the same gene, e.g. brown or blue eye colour.
- Meiosis
- Cell division that produces four genetically different haploid gametes (23 chromosomes in humans).
- Genotype vs phenotype
- Genotype = the alleles present (e.g. Bb); phenotype = the physical characteristic shown (e.g. brown eyes).
- Humans have 23 pairs of chromosomes; the 23rd pair determines sex (XX female, XY male).
- Dominant alleles are shown even when only one copy is present; recessive need two copies.
- Variation arises from mutation, meiosis and sexual reproduction combining two parents' DNA.
Worked example
Two parents are heterozygous for brown eyes (Bb × Bb). Predict the ratio of offspring phenotypes.
- 1Draw a Punnett square with Bb across the top and Bb down the side.
- 2Fill in: BB, Bb, Bb, bb.
- 3Genotype ratio 1 BB : 2 Bb : 1 bb.
- 4Phenotype ratio 3 brown eyes : 1 blue eyes.
Answer: 3 brown : 1 blue eyes
Common mistakes
✗ Confusing genotype (letters) with phenotype (feature).
✓ Genotype = Bb; phenotype = brown eyes.
✗ Writing the recessive allele as capital.
✓ Dominant = capital (B); recessive = lowercase (b).
✗ Using mitosis instead of meiosis for gametes.
✓ Gametes are made by meiosis — halving chromosome number.
Quick check
Try these, then reveal the answers.
How many chromosomes are in a human gamete?Reveal answer
23 (haploid).
What does XY code for?Reveal answer
A male.
If both parents are heterozygous (Bb), what fraction of offspring show the recessive phenotype?Reveal answer
1 in 4 (25%).
Exam tips
- Draw a clear Punnett square — label parents, gametes and every cell.
- State ratios in the correct order for the phenotypes asked (usually dominant : recessive).
- Distinguish mitosis (identical, body cells) from meiosis (variation, gametes).
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Frequently asked questions
What is the difference between a gene and an allele?+
A gene is a section of DNA coding for a protein; alleles are the different versions of that gene.
Why is meiosis important for evolution?+
It produces genetically varied gametes; combined with mutation this creates the variation natural selection acts on.
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