Explaining Change: Inheritance — AQA Combined Science: Synergy
This topic covers chromosomes and genes, sex determination, single-gene inheritance and the link between genotype and phenotype.
Chromosomes and genes
- DNA is the molecule that carries the genetic code, arranged as a double helix and contained in chromosomes in the nucleus.
- A gene is a section of DNA that codes for a particular protein.
- The genome is the entire genetic material of an organism. Studying it helps identify genes linked to disease and trace human ancestry.
Humans have 23 pairs of chromosomes in body cells. Gametes (made by meiosis) have a single set of 23.
Sex determination in humans
One pair of chromosomes determines sex:
- Females are XX, males are XY. A Punnett square of X and Y shows a 50% chance of each sex at fertilisation.
Single gene inheritance
Key terms:
- Allele — a version of a gene.
- Dominant — shows with one copy (capital letter).
- Recessive — needs two copies (lower-case).
- Homozygous — two identical alleles; heterozygous — two different alleles.
Use a Punnett square to predict offspring and express results as ratios or probabilities. For example, crossing two heterozygous parents (Bb × Bb) gives a 3:1 ratio of dominant to recessive phenotypes.
Inherited disorders
- Polydactyly (extra digits) — caused by a dominant allele.
- Cystic fibrosis — caused by a recessive allele (needs two copies).
Genotype and phenotype
- Genotype — the alleles an organism has.
- Phenotype — the characteristics that are shown.
The phenotype can be affected by the genotype, the environment, or both. Most characteristics are controlled by several genes working together, not single genes.
Exam tips
- Remember females are XX, males XY, with a 50:50 chance of each sex.
- Practise Punnett squares and converting outcomes to ratios and percentages.
- Polydactyly is dominant; cystic fibrosis is recessive.
- Distinguish genotype (alleles) from phenotype (characteristics shown).