HookThe photograph that gave away the shape of life
In May 1952, in a basement laboratory at King's College London, a physical chemist named Rosalind Franklin used a fine beam of X-rays to photograph a single fibre of purified DNA. The image, catalogued as Photograph 51, showed a blurred black cross of spots. To a trained eye it was unmistakable: DNA was a helix, its dimensions written directly in the spacing of the marks. When James Watson and Francis Crick saw the photograph the following year, the last piece fell into place, and in April 1953 they published the structure of the DNA double helix — two strands wound around each other, carrying a chemical code. It was arguably the most important photograph ever taken, because it revealed the molecule that copies itself every time a cell divides and passes life's instructions from one generation to the next.
And yet the rules of inheritance had been worked out almost a century earlier, by a monk counting peas, long before anyone knew DNA existed. That gap — knowing how traits pass on before knowing the molecule that carries them — is the story of this whole section. B6 runs from how organisms reproduce and how gametes are made, through the structure of DNA and the mathematics of a genetic cross, to the biggest idea in biology: that all of this variation, passed on and filtered over vast time, is how life evolves. By the end you should be able to fill in a Punnett square, explain natural selection as a chain of cause and effect, and say why a bacterium in a British hospital and a finch on a Pacific island are telling the same story.
ModelTwo ways to reproduce — and meiosis in the middle
Sexual reproduction involves two parents and the joining of gametes — sex cells — at fertilisation: the egg and sperm in animals, egg cells and pollen in flowering plants. Because each offspring gets a mix of genes from two parents, sexual reproduction produces variation. Asexual reproduction involves only one parent and no gametes; the offspring are genetically identical clones, produced by ordinary cell division (mitosis).
Gametes are made by a special kind of cell division called meiosis, which happens only in the reproductive organs. Meiosis starts with a normal body cell and copies the chromosomes, then divides twice to form four gametes, each with only half the chromosome number — in humans, 23 instead of 46 — and each genetically different from the others. When two gametes fuse at fertilisation, the full number is restored, and the new cell divides by mitosis to build an embryo. Halving then doubling is what keeps the chromosome number constant across generations while shuffling the genes.
Each strategy is a trade-off. Sexual reproduction creates variation, which gives a population a survival advantage if the environment changes and is the raw material selective breeding exploits — but it needs two parents, takes time and energy, and requires them to meet. Asexual reproduction is fast, needs only one parent and is energy-efficient in a stable habitat — but with no variation, a single disease or change can wipe out the whole population. Many organisms hedge by doing both: a strawberry plant sends out runners (asexual) and also flowers (sexual); the malarial parasite reproduces asexually in humans and sexually in mosquitoes.
ModelDNA, the genome and the double helix
The genetic material is DNA, a polymer twisted into a double helix and contained in structures called chromosomes in the nucleus. A gene is a small section of DNA that codes for a particular sequence of amino acids, which fold to make a specific protein. The entire genetic material of an organism is its genome. Working out the whole human genome has been vital in medicine — it lets scientists search for genes linked to disease, understand and treat inherited disorders, and trace how human populations migrated across the world.
Higher-tier, biology-only students go one level deeper into structure. DNA is made of repeating units called nucleotides, each a sugar and a phosphate group with one of four bases attached, abbreviated A, C, G and T. The two strands are held together by pairs of bases, and the pairing is fixed and complementary: A always pairs with T, and C always pairs with G. It is the order of these bases along a gene that spells out the order of amino acids in a protein — a code of four letters, read in threes, building the tens of thousands of proteins that make a body work.
MechanismReading a genetic cross — alleles and Punnett squares
Genes come in different versions called alleles. Because you inherit one chromosome of each pair from each parent, you carry two alleles of every gene. If the two alleles are the same you are homozygous; if they differ you are heterozygous. A dominant allele shows its effect even when only one copy is present (written with a capital letter); a recessive allele only shows its effect when both copies are present (a lower-case letter). Your combination of alleles is your genotype; the characteristic it produces is your phenotype.
A Punnett square is a grid that predicts the offspring of a cross. You write the two alleles of one parent along the top and the two of the other down the side, then fill each cell by combining them. The proportions it gives are probabilities for each offspring, not guarantees — a family of four can easily not match the ratio exactly, because each conception is an independent event.
Cystic fibrosis is caused by a recessive allele. Write the healthy dominant allele as F and the cystic-fibrosis recessive allele as f. Two parents who each carry one copy — genotype \(Ff\), so healthy but 'carriers' — have a child. The cross is \(Ff \times Ff\). Each parent passes on either F or f, so the Punnett square combines them into four equally likely outcomes: FF, Ff, Ff and ff. That gives a genotype ratio of \(1:2:1\) and a phenotype ratio of unaffected to affected of \(3:1\). So for each child the probability of having cystic fibrosis (genotype ff) is \(\tfrac{1}{4}\), or \(25\%\); the probability of being a healthy carrier (Ff) is \(\tfrac{1}{2}\); and the probability of being completely unaffected and not a carrier (FF) is \(\tfrac{1}{4}\). The key point for full marks: each child is an independent \(\tfrac{1}{4}\) risk — having one affected child does not 'use up' the odds for the next.
CaseInherited disorders and sex determination
Some disorders are caused by the inheritance of certain alleles. Polydactyly — having extra fingers or toes — is caused by a dominant allele, so it can be passed on by just one affected parent. Cystic fibrosis — a disorder of cell membranes that clogs the lungs and pancreas with thick mucus — is caused by a recessive allele, so a child can only have it by inheriting a copy from both parents, who may themselves be unaffected carriers. Embryos produced by IVF can be screened for such alleles before implantation, which raises ethical questions AQA expects you to weigh: it could reduce suffering, but it also raises fears about rejecting embryos, cost, and where to draw the line on selecting for characteristics.
Sex is determined by one pair of chromosomes, the 23rd. Females have two X chromosomes (XX); males have one X and one Y (XY). Every egg carries an X; a sperm carries either an X or a Y. A Punnett cross of \(XX \times XY\) gives XX, XX, XY, XY — a \(1:1\), or 50:50, ratio of females to males, which is why the sex ratio at birth is close to even.
MechanismVariation and natural selection
Differences between individuals of the same species are called variation, and they arise from the genes they inherit (genetic causes), from their environment (environmental causes), or from a combination of the two. All genetic variation originates in mutation — a random change to the DNA. Most mutations have no effect on the phenotype; a few influence it slightly; and very rarely, a single mutation determines a phenotype outright. Mutations occur continuously, giving natural selection a constant supply of new variation to act on.
Natural selection is the mechanism of evolution, and examiners want it as an ordered chain of cause and effect. Within a species there is genetic variation. Organisms compete for limited resources, and more offspring are produced than can survive. Individuals with characteristics best suited to the environment are more likely to survive this struggle and reproduce — 'survival of the fittest'. They pass the alleles for those advantageous characteristics on to their offspring. Over many generations, those alleles become more common in the population, and the species gradually changes. That is the definition of evolution: a change in the inherited characteristics of a population over time, through the process of natural selection, which may result in the formation of a new species.
CaseHumans take the wheel — breeding, engineering, cloning
Selective breeding (artificial selection) is natural selection with a human doing the choosing. Over many generations, breeders pick the individuals with the desired characteristic — disease resistance or high yield in crops, high milk production in cattle, gentleness in dogs, large or unusual blooms in flowers — and breed them together. Its danger is a shrinking gene pool: repeatedly breeding closely related individuals (inbreeding) can accumulate harmful alleles and leave a whole breed vulnerable to a single disease.
Genetic engineering goes further and faster by transferring a gene directly from one organism into another. Bacteria have been engineered to make human insulin; crops (GM crops) have been given genes for herbicide tolerance, pest resistance or added nutrients — 'golden rice' carries genes that make vitamin A precursors. The debate examiners want weighed: higher yields and medical benefits against uncertainty about effects on wild populations, human health and ethics.
Cloning (biology only) makes genetically identical copies. Simple methods include taking cuttings from a plant and growing thousands of identical plants from tiny samples by tissue culture. In animals, an embryo can be split and the cells transplanted into host mothers. The most advanced method is adult cell cloning — the technique behind Dolly the sheep, produced at the Roslin Institute near Edinburgh in 1996: the nucleus is removed from an unfertilised egg and replaced with the nucleus of an adult body cell, and the egg is stimulated to divide and grow into a clone of the adult.
CaseThe idea itself — Darwin, Lamarck and Mendel
Charles Darwin proposed the theory of evolution by natural selection in On the Origin of Species in 1859, after years of observation on the voyage of the Beagle and decades of study. Alfred Russel Wallace arrived at the same idea independently and the two published jointly, prompting Darwin to finish his book. The theory was accepted only gradually. Three reasons AQA wants you to give: it challenged the religious belief that God made all the animals and plants; there was insufficient evidence at the time to convince many scientists; and, crucially, the mechanism of inheritance — genes and how they pass on — was not understood for another fifty years, so Darwin could not explain how variation was inherited.
An earlier idea, from Jean-Baptiste Lamarck, held that characteristics an organism develops during its life — a blacksmith's strong arm, a giraffe's stretched neck — are passed to its offspring. This 'inheritance of acquired characteristics' was rejected because such changes to the body do not alter the genes in the gametes, and experiments showed acquired traits are not inherited.
The missing mechanism came from Gregor Mendel, an Austrian monk who, in the 1860s, bred thousands of pea plants and worked out that inheritance follows fixed mathematical rules, carried by discrete 'units' — what we now call genes. His work was overlooked for decades and only recognised, long after his death, once chromosomes and DNA were discovered and shown to behave exactly as his units predicted.
DataThe evidence — fossils, resistance, speciation and extinction
Two main lines of evidence support evolution: the fossil record and, more recently, our understanding of genetics. Fossils are the remains of organisms from thousands or millions of years ago, formed when hard parts like bone do not decay and are gradually replaced by minerals, when softer parts are preserved because conditions (very low oxygen, temperature or water) stop microbes decaying them, or as impressions and traces such as footprints and burrows. Scientists cannot be certain how life began because many early organisms were soft-bodied and left few fossils, and many fossils that did form have been destroyed by geological activity — leaving gaps in the record.
The most powerful modern evidence is evolution you can watch: antibiotic-resistant bacteria such as MRSA. A random mutation gives a bacterium resistance to an antibiotic. When the antibiotic is used, non-resistant bacteria die but the resistant one survives and reproduces rapidly, passing on the resistance allele, until the whole population is resistant — natural selection in days, not aeons. To slow it, doctors are urged not to over-prescribe antibiotics, to prescribe specific ones, and patients must complete the full course; and antibiotics should be restricted in agriculture. New antibiotics are developed slowly and expensively, so we cannot rely on outrunning resistance.
When two populations of a species become isolated — geographically, for instance — and experience different conditions, natural selection changes them in different directions until they can no longer interbreed to produce fertile offspring. At that point they are separate species: this is speciation, an idea first proposed by Wallace. And when a species can no longer survive the conditions it faces — through environmental change, new predators, new diseases, new competitors, a single catastrophic event, or human activity — it becomes extinct.
ModelClassification — from Linnaeus to three domains
Living things must be organised to be studied. In the eighteenth century Carl Linnaeus classified organisms into a nested hierarchy — kingdom, phylum, class, order, family, genus, species — and gave each a two-part Latin name in the binomial system, genus then species, such as Homo sapiens. As microscopes and then biochemistry improved, scientists could see far more detail than Linnaeus could, and the scheme was revised.
In the 1970s and beyond, Carl Woese used comparisons of RNA and other molecules to propose the three-domain system, dividing life at its broadest level into Archaea (primitive bacteria often from extreme environments), Bacteria (true bacteria) and Eukaryota (everything with complex cells — protists, fungi, plants and animals). Because classification now reflects how closely related organisms are, it can be drawn as evolutionary trees that show how species have diverged from common ancestors — the same idea of shared descent that runs through the whole of B6.
VocabularyKey terms the mark scheme pays for
TrapsMisconceptions that cost marks
ExamWhat examiners want
Genetic-cross questions are pure method marks: draw the full Punnett square, label the parental genotypes and the gametes, and quote both the ratio and the probability (as a fraction or percentage). State that each offspring is an independent event — the single most common lost mark on inheritance questions. Use the vocabulary with precision, because AQA hands marks for distinguishing gene from allele, genotype from phenotype, and homozygous from heterozygous.
For natural selection, always answer as an ordered chain: variation from mutation → competition and a selection pressure → the best-suited survive and reproduce → they pass on their alleles → the allele becomes more common over many generations. Stress 'populations, over many generations' — never individuals changing themselves. For antibiotic-resistance and speciation questions this same chain, applied to a specific example, is the mark scheme.
On the history-of-science questions, learn the set answers: the three reasons Darwin's theory was accepted slowly (religion, insufficient evidence, no known mechanism of inheritance), why Lamarck was rejected (acquired characteristics do not change the genes in gametes), and why Mendel was recognised only after his death (chromosomes and DNA had not yet been discovered). These recall marks are the easiest in the paper to bank if you have the phrasing ready.