Inheritance — alleles, Punnett squares and inherited disorders
Inheritance is the passing of features from parents to offspring through genes. The classic Mendelian patterns examined at GCSE all come from understanding alleles, genotypes and phenotypes.
📖Definition— Key terms — get these word-perfect
- Gene — a section of DNA that codes for a protein.
- Allele — a different version of a gene (e.g. brown-eye allele vs blue-eye allele).
- Genotype — the combination of alleles an organism has for a gene (e.g. Bb).
- Phenotype — the actual physical characteristic shown (e.g. brown eyes).
- Homozygous — two identical alleles (BB or bb).
- Heterozygous — two different alleles (Bb).
- Dominant allele (capital letter) — expressed even if only one copy is present.
- Recessive allele (small letter) — only expressed if both copies are recessive.
Punnett squares — the standard exam tool
A monohybrid cross uses a 2 × 2 grid showing all possible combinations of parental gametes.
Example: Bb × Bb (both parents heterozygous, B = brown, b = blue eyes).
| B | b | |
|---|---|---|
| B | BB | Bb |
| b | Bb | bb |
Genotype ratio: 1 BB : 2 Bb : 1 bb (1:2:1). Phenotype ratio: 3 brown : 1 blue (3:1). Probability that any one child has blue eyes: ¼ = 25 %.
Sex determination
Humans have 23 pairs of chromosomes. The 23rd pair are the sex chromosomes:
- Female: XX
- Male: XY
Sperm carry either X or Y, so the father determines the sex of the child. A 2 × 2 Punnett shows there's a 50 % chance of each sex.
Inherited disorders (you must know these)
Cystic fibrosis (CF)
- Caused by a recessive allele on chromosome 7.
- Patients (genotype ff) produce thick, sticky mucus that clogs lungs and the pancreas.
- Two heterozygous (carrier) parents (Ff × Ff) give a 1 in 4 (25 %) chance of an affected child.
- Carriers (Ff) show no symptoms.
Polydactyly
- Extra fingers or toes.
- Caused by a dominant allele.
- Only one copy needed → if a parent has it, each child has a 50 % chance.
These two diseases are great examples to compare dominant vs recessive inheritance.
Embryo screening
In IVF (B5.9), one or two cells from an early embryo can be tested for genes linked to inherited diseases (pre-implantation genetic diagnosis, PGD). Embryos that don't carry harmful alleles are then selected to be implanted.
Arguments for screening:
- Reduces suffering caused by inherited disease.
- Saves potential costs of long-term treatment.
- Laws and regulators control its use to prevent misuse.
Arguments against:
- Could lead to "designer babies" — choosing characteristics like eye colour or sex.
- Discrimination against people with disabilities.
- Expensive and emotionally demanding.
Probability vs predicted ratios
Punnett ratios are predictions of probability, not guarantees. With small numbers of offspring you can get any combination — the predicted 3:1 only emerges in large numbers, just like coin flips average 50:50 over many tosses but not in any 4 throws.
⚠Common mistakes
- Mixing up alleles and genes. A gene is the position; alleles are the versions.
- Capitalising recessive alleles. Use little letters for recessive — it's how examiners test you.
- Saying "the child WILL have blue eyes". Punnett squares give probabilities; each child is independent.
- Confusing genotype and phenotype. BB and Bb give the same phenotype (brown) but different genotypes.
Links
Builds on B6.1 (gametes, fertilisation) and B6.2 (genes/DNA). Polydactyly and CF lead into B6.5 (variation/mutation) and B6.7 (genetic engineering as a future treatment).
AI-generated · claude-opus-4-7 · v3-deep-biology