Inheritance (B5.1)
Genetics is one of the most reliably examined Gateway A topics. Expect a Punnett square question, probability calculations, a question on sex determination and likely a question on cystic fibrosis or another inherited disorder.
📖Definition— Key terminology
| Term | Definition |
|---|---|
| Gene | A section of DNA coding for a protein; determines a characteristic |
| Allele | A version of a gene (e.g. the tall allele or the dwarf allele of the height gene) |
| Genotype | The alleles an organism carries (e.g. Tt) |
| Phenotype | The observable characteristic (e.g. tall) |
| Dominant | An allele that is expressed when only ONE copy is present (written in upper case: T) |
| Recessive | An allele only expressed when TWO copies are present (lower case: t) |
| Homozygous | Both alleles identical (TT or tt) |
| Heterozygous | Two different alleles (Tt) |
| Carrier | Heterozygous for a recessive disorder — has one copy of the recessive allele but does not show the disorder |
Punnett squares
Used to predict the ratios of offspring genotypes and phenotypes from a cross.
Example: tall (Tt) × tall (Tt)
| T | t | |
|---|---|---|
| T | TT | Tt |
| t | Tt | tt |
Genotype ratios: 1 TT : 2 Tt : 1 tt Phenotype ratios: 3 tall : 1 dwarf (because T is dominant) Probability of dwarf offspring: 1 in 4 = 25%
Sex determination
Humans have 23 pairs of chromosomes. Pair 23 are the sex chromosomes:
- XX = female
- XY = male
All eggs carry X. Sperm can carry X or Y. So sex is determined by the sperm. A Punnett cross:
| X (mother) | X (mother) | |
|---|---|---|
| X (sperm) | XX (girl) | XX (girl) |
| Y (sperm) | XY (boy) | XY (boy) |
Ratio: 1:1 male:female — every pregnancy is a 50% chance of each sex.
Monohybrid inheritance — worked examples
Cystic fibrosis (CF)
- Caused by a recessive allele (f).
- A person must be homozygous ff to have the disease.
- Ff = carrier (healthy but can pass on the allele).
- FF = unaffected (no copies).
Two carrier parents (Ff × Ff):
| F | f | |
|---|---|---|
| F | FF | Ff |
| f | Ff | ff |
Probability of child having CF: 1 in 4 = 25%. Probability of child being a carrier: 2 in 4 = 50%.
Polydactyly (extra fingers/toes)
- Caused by a dominant allele (D).
- Any person with at least one D allele shows the condition.
- Dd (heterozygous) still has polydactyly.
Predicted ratios vs actual ratios
Punnett squares give predicted probabilities. Actual ratios in small families will differ because inheritance is random. The larger the sample, the closer to the theoretical ratio.
Common Gateway-paper mistakes
- Forgetting to write out the parent genotypes above the Punnett grid.
- Writing ratios as fractions instead of converting to percentages when asked for probability.
- Confusing "carrier" with "affected" for recessive disorders.
- Saying dominant traits are "stronger" or "better" — dominant just means expressed with one copy.
- Not labelling the cross (e.g. F₁ generation).
AI-generated · claude-opus-4-7 · v3-ocr-combined-science