DNA structure, Punnett squares, inherited disorders, sex determination and genetic engineering — all of GCSE genetics in one place.
Genetics is one of the most concept-heavy topics in GCSE Biology. It requires you to understand a chain of ideas — from the structure of DNA, through how genes code for proteins, to how alleles are inherited across generations — and exam questions test this chain at multiple levels. This guide builds that understanding from the ground up.
DNA (deoxyribonucleic acid) is a double helix — two strands wound around each other, held together by pairs of bases. There are four bases: adenine (A), thymine (T), cytosine (C) and guanine (G). They pair in a specific way: A always pairs with T, and C always pairs with G. This complementary base pairing is fundamental to how DNA replicates and how it codes for proteins.
A gene is a specific sequence of bases on a DNA molecule. Each sequence codes for a particular protein by specifying which amino acids should be joined together and in what order. Since proteins control everything that happens in a cell — including which features develop — genes ultimately determine an organism's characteristics.
The full set of DNA in an organism is called its genome. Human cells (apart from red blood cells) contain the full genome, arranged into 23 pairs of chromosomes. One chromosome from each pair came from the mother's egg, the other from the father's sperm.
Genetics questions are lost and won on precise use of terminology. These definitions must be exact.
❌ Most common mistake: confusing genotype and phenotype. A student who is heterozygous (Bb) for brown eyes has the genotype Bb but the phenotype "brown eyes" — because B is dominant. Never write the genotype when asked for the phenotype, and vice versa.
A Punnett square predicts the probability of offspring inheriting different genotypes. It's a grid showing all possible combinations of alleles from the two parents.
Example: Both parents are heterozygous for cystic fibrosis (Ff, where f is the recessive allele for cystic fibrosis).
Set up the grid: write F and f across the top (father's alleles) and F and f down the side (mother's alleles). Fill in each cell by combining the allele from that row with the allele from that column.
Results: FF (25%), Ff (50%), ff (25%). The ff offspring will have cystic fibrosis. The FF and Ff offspring will not — but Ff individuals are carriers who can pass the allele to their children.
Punnett squares give probabilities — not certainties. A 25% chance of having an affected child doesn't mean one in every four children will be affected. Each pregnancy is an independent event with the same 25% probability. This distinction is tested directly in exam questions.
Sex in humans is determined by the sex chromosomes. Females have two X chromosomes (XX). Males have one X and one Y chromosome (XY). All eggs carry an X chromosome. Sperm carry either an X or a Y.
If a Y-carrying sperm fertilises the egg: XY — male. If an X-carrying sperm fertilises the egg: XX — female. The probability of each outcome is 50:50 for every conception.
Exam questions sometimes ask you to draw a Punnett square for sex determination. The setup is exactly the same as any other cross — X and Y across the top (father), X and X down the side (mother). The offspring genotypes are XX, XX, XY, XY — so 50% female, 50% male.
Two inherited disorders appear consistently in GCSE Biology questions: cystic fibrosis and polydactyly. You need to know both.
Caused by a recessive allele (f). Both copies of the allele must be present for the disorder to occur (genotype ff). People with one copy (Ff) are carriers — they have no symptoms but can pass the allele on. The condition causes thick, sticky mucus to build up in the lungs and digestive system, making breathing difficult and reducing nutrient absorption.
Caused by a dominant allele (P). Only one copy is needed for the condition to occur. A person with genotype PP or Pp will have polydactyly — having extra fingers or toes. Because the allele is dominant, it cannot be "hidden" in carriers. If a parent has polydactyly, there is a 50% chance of each child being affected (assuming the parent is heterozygous Pp, which is the most common genotype).
Selective breeding (also called artificial selection) is the process of choosing individuals with desirable characteristics and breeding them together over many generations to enhance those characteristics in the population.
Examples include breeding cows for higher milk yield, wheat for disease resistance, or dogs for temperament. The process works by exploiting natural genetic variation — selecting individuals that already carry the alleles for desired traits and allowing only them to reproduce.
The disadvantage is reduced genetic diversity. If only a small number of individuals with similar genotypes are bred, the population becomes genetically uniform. This makes the species vulnerable — if a new disease emerges that the uniform genome cannot resist, the entire population could be wiped out.
Genetic engineering involves directly inserting a gene from one organism into the genome of another. The inserted gene is called a transgene, and the organism that receives it is described as transgenic or genetically modified (GM).
The most cited example at GCSE is the production of human insulin. The gene for human insulin is cut from human DNA using enzymes called restriction enzymes. The gene is then inserted into a bacterial plasmid (a small circular piece of DNA) using another enzyme called ligase. The modified plasmid is inserted into a bacterial cell, which then replicates rapidly and produces human insulin in large quantities. This insulin is used to treat Type 1 diabetes.
Restriction enzymes cut DNA at specific sequences — they are the molecular scissors. Ligase joins DNA strands together — it is the molecular glue. You need both names and functions for exam questions on genetic engineering. "An enzyme cuts the gene out" is not specific enough to score the mark — you need to say restriction enzyme.
The AQA Biology genetics specification is detailed on the AQA GCSE Biology page. OCR's equivalent is on the OCR GCSE Biology page.
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