GCSE Biology Inheritance revision

Most body cells carry two alleles for every gene — one on each chromosome of the pair, one from each parent. Alleles are versions of the same gene. A dominant allele shows its effect even with only one copy present; a recessive allele is hidden unless both alleles are recessive. Convention writes dominant alleles as capitals (B) and recessives as lower case (b).

Your genotype is the combination of alleles you carry — BB, Bb or bb. Your phenotype is the characteristic you actually show. BB and Bb both show the dominant characteristic; only bb shows the recessive one. Homozygous means both alleles are the same (BB or bb); heterozygous means one of each (Bb).

A Punnett square lays out every possible allele combination from two parents, letting you predict the probability of each offspring type. It predicts odds, not certainties — a 25% chance says nothing about any single child, only about the long-run average.

Reading the square: 1 BB : 2 Bb : 1 bb. Phenotypes: 3 dominant : 1 recessive — a 75% chance of showing the dominant characteristic and a 25% chance of the recessive one. Crossing two homozygous parents (BB × bb) gives all heterozygous offspring (100% dominant phenotype). Crossing a heterozygote with a homozygous recessive (Bb × bb) gives a 1:1 ratio — 50:50.

A family tree (pedigree) plots a characteristic across generations — circles for females, squares for males, shading for those showing the trait. Two clues tell you the inheritance mode: if two unaffected parents have an affected child, the trait is recessive (the parents must be carriers); if two affected parents have an unaffected child, the trait is dominant (that child inherited two recessive alleles).

Once the mode is known, genotypes in the tree can be worked out and probabilities for future children calculated — exactly the exam skill family-tree questions test.

Of the 23 chromosome pairs in body cells, 22 are matched pairs and one pair — the sex chromosomes — decides sex. Females carry XX; males carry XY. All eggs carry an X. Sperm carry either an X or a Y, so the sperm that fertilises the egg decides: XX gives a female, XY gives a male.

The square shows a 1:1 ratio — every pregnancy has a 50% probability of a girl and 50% of a boy, independent of previous children.

Polydactyly — extra fingers or toes — is caused by a dominant allele. It can be inherited from just one affected parent, who passes it to 50% of children (if heterozygous). Cystic fibrosis — a disorder of cell membranes producing thick, sticky mucus that affects the lungs and digestion — is caused by a recessive allele. Both parents must pass on the allele; unaffected carriers (heterozygous) show no symptoms but can pass the allele on.

Embryo screening can test embryos for these disorders before birth or implantation. The evaluation has real trade-offs: screening gives families information and choices — preparing for care, or deciding whether to continue a pregnancy — but raises ethical issues (deciding which embryos are acceptable, the fate of screened-out embryos), economic costs of testing and treatment, and social concerns about where screening should stop — detecting disorders, or choosing other characteristics.

In the mid-19th century, the Austrian monk Gregor Mendel bred pea plants and counted offspring characteristics — tall versus short, smooth versus wrinkled. His ratios (always neat fractions like 3:1) showed that characteristics are inherited through discrete units — what we now call genes. His work was largely ignored for decades until chromosomes were observed and the units were matched to them; Mendel was recognised only long after his death.

Variation in a population comes from mutations — new alleles appearing — and from the reshuffling of alleles in sexual reproduction. The phenotype an organism finally shows is the interaction of genotype and environment: genes may set your height potential, but diet and childhood health decide how much of it you reach; body mass responds strongly to diet and exercise; natural hair colour is almost purely genetic. Exam answers score by naming both contributors.