4.6 Inheritance, variation and evolution

Unit Summary

This unit introduces inheritance, variation and evolution: how genetic information is passed between generations, why offspring differ from their parents, and how these differences drive the evolution of species over time, and is fully aligned with the AQA GCSE Combined Science: Trilogy specification. It builds on prior knowledge of cells and DNA structure, and prepares students for later topics such as ecology, where variation and adaptation are revisited in the context of whole ecosystems.

Students first compare asexual and sexual reproduction, learning that asexual reproduction involves only one parent and produces genetically identical offspring, while sexual reproduction involves the fusion of male and female gametes to produce genetically varied offspring. This leads into meiosis, the type of cell division that produces gametes with half the normal number of chromosomes, so that fertilisation restores the full chromosome number while combining genetic material from both parents. Students then study sex determination and how the X and Y chromosomes decide whether offspring are male or female.

Students then study DNA and the genome, understanding that genetic information is stored in the genome within the nucleus and that the genome as a whole can be studied to understand inherited disorders and human evolution.

Genetic inheritance is taught next, using genetic diagrams and Punnett squares to predict the outcome of monohybrid crosses, introducing key terms such as allele, genotype, phenotype, homozygous, heterozygous, dominant and recessive. Inherited disorders such as polydactyly and cystic fibrosis are studied as examples of how alleles are passed through family pedigrees.

Students study genetic engineering as a technique for transferring a gene from one organism into the genome of another, even across species, with applications such as producing human insulin in bacteria and disease-resistant crops.

The unit then turns to variation, where students learn that gene mutations occur continuously and, while most have no effect, some are damaging and some are rarely beneficial, increasing an individual's fitness. This variation, combined with that produced by sexual reproduction, is the basis for evolution, the gradual change in a species' characteristics over generations.

Students then examine the evidence for evolution, first through fossils and what they reveal about organisms that lived millions of years ago, and then through resistant bacteria, which provide direct, observable evidence of natural selection and evolution happening within a human timescale. This leads into extinction, where students study the range of pressures that can drive a species to extinction, including new predators or competitors, new diseases, environmental change, habitat destruction, catastrophic events and hunting.

Students then study how humans have applied this understanding of genetics and variation directly through selective breeding, the deliberate selection and breeding of organisms with desirable characteristics over many generations, with examples from food crops, farm animals and domestic dogs, alongside the risks of reduced genetic variation and inbreeding.

The unit closes with the classification of living organisms, covering the Linnaean system of kingdom, phylum, class, order, family, genus and species, and its more recent replacement by the three-domain system proposed by Carl Woese, which divides organisms into Archaea, Bacteria and Eukaryota based on evolutionary relationships revealed by biochemical evidence.

This unit has no required practical of its own, but draws on practical and evaluative skills developed earlier in the course, such as interpreting data and weighing evidence. Real-world links include selective breeding of livestock and crops, the use of genetic engineering in medicine and agriculture, and the causes and consequences of species extinction.

To promote deep and long-term learning, a variety of Assessment for Learning (AfL) strategies are woven into lessons, including retrieval practice, diagnostic questioning, model evaluation, and structured discussion. These approaches reinforce conceptual understanding, promote metacognition, and encourage students to connect ideas across the curriculum.