AQA GCSE Biology (8461) · Paper 2
🧬 Inheritance & Evolution
Revision notes written to the specification, with examiner tips and the required practicals. Every point here has flashcards in the Stickwise app.
Reproduction
Gametes are made by meiosis, a special type of division in the reproductive organs that halves the chromosome number, from 46 to 23 in humans, and produces four genetically different cells. At fertilisation, two gametes fuse and restore the full number of 46 chromosomes, half from each parent.
Triple content: sexual reproduction produces variation, which gives natural selection material to act on and allows selective breeding; asexual reproduction is faster and more energy-efficient, since it needs only one parent, which is an advantage when conditions are stable. Some organisms use both methods: malaria parasites reproduce asexually in humans and sexually in mosquitoes, and many fungi and plants, such as strawberries, which spread by runners as well as flowering, do the same.
DNA, genes and the genome
Triple content: DNA is a polymer made of nucleotides. Each nucleotide is built from a sugar, a phosphate and one of four bases: A, T, C and G. The sugar and phosphate groups alternate along each strand to form its backbone. A sequence of three bases codes for one amino acid. The order of bases determines the order of amino acids in a particular protein.
Higher content (Triple): across the two strands, base A always pairs with T and base C always pairs with G. Mutations change bases all the time; most change nothing, but a few alter the protein's shape, for example so that an enzyme's active site no longer fits its substrate.
Genetic crosses
Two heterozygous parents, Bb crossed with Bb, produce offspring in a 3:1 ratio showing the dominant trait to the recessive trait, so there is a 25% chance of the recessive phenotype. Two named conditions are commonly examined. Polydactyly, which causes extra fingers, is caused by a dominant allele, so a single affected parent can pass it on. Cystic fibrosis, which causes thick mucus in the lungs and digestive system, is recessive, so both parents can be unaffected carriers, with the genotype Ff.
Family trees can reveal carriers: if an affected child, genotype ff, is born to two unaffected parents, both parents must be carriers with the genotype Ff. Embryo screening can test embryos for such alleles before implantation. Arguments for screening include preventing suffering and saving the cost of long-term treatment; arguments against include the destruction of embryos, the expense, and concern that it could lead towards so-called 'designer babies'.
Biological sex is determined by the 23rd chromosome pair: XX produces a female and XY produces a male. A Punnett square crossing X and Y chromosomes gives the familiar 50:50 chance of each sex.
Variation and evolution
Differences within a species are called variation, and they can come from genes, which are inherited, from the environment, such as a suntan or a scar, or from a combination of both, such as height. New alleles arise only by mutation; usually they change nothing, but occasionally one gives a real advantage, and if the environment changes, that allele can spread through a species surprisingly quickly.
Evolution by natural selection
Evolution is a change in a species' inherited characteristics over time. Darwin's mechanism of natural selection works in steps:
- Individuals in a species show wide variation, including variation caused by mutations.
- There is competition for food, mates and survival.
- The best-suited variants are more likely to survive and breed, sometimes called 'survival of the fittest'.
- Their alleles pass on to a greater number of offspring.
- Over many generations, this process repeats and the species gradually changes.
All life evolved from simple organisms over approximately 3 billion years. When two populations of one species change so much that they can no longer interbreed to produce fertile offspring, they have become separate species, a process called speciation.
Humans steering inheritance
Higher content: in genetic engineering the wanted gene is cut out using enzymes and inserted into a vector, such as a bacterial plasmid or a virus, which carries it into the cells of the other organism at an early stage of development.
Concerns about genetically modified organisms include possible effects on wild ecosystems, such as genes spreading to weeds, unknown long-term health effects, although there is no strong evidence of harm so far, and control of the seed supply by large companies. These concerns must be balanced against the benefits of higher yields and vitamin-enriched food.
Triple content covers four cloning methods. Cuttings and tissue culture are used for plants, in which tiny groups of cells are grown on agar jelly to produce thousands of identical plants quickly, which also helps protect rare species. Embryo transplants split an early animal embryo and implant the resulting halves into host mothers, producing identical twins or quadruplets. Adult cell cloning takes the nucleus from a body cell and inserts it into an egg cell with its own nucleus removed; a small electric shock starts cell division, and the embryo is then implanted, as was done to produce Dolly the sheep.
Development of understanding
Triple content covers the history of this idea. Lamarck thought that characteristics acquired during an organism's life, such as a stretched neck, could be passed on to offspring; this idea was wrong, but it was a serious early attempt to explain evolution. Darwin, together with Wallace, who independently reached the same idea and co-published it in 1858, proposed natural selection, publishing *On the Origin of Species* in 1859. Acceptance of the theory was slow, because it clashed with religious views, Darwin could not explain how traits were inherited, since genetics was not yet understood, and the fossil evidence at the time was thin.
Triple content: Mendel, a monk who bred pea plants in the 1860s, showed that inheritance is passed on in discrete units, now known as genes, which behave in dominant and recessive ways. This was decades before anyone knew what DNA was, so the importance of his work was not recognised until after his death.
Evidence for evolution
Fossils are the remains of organisms preserved in rock. They can form when minerals replace hard body parts, from casts and impressions, or from bodies preserved in places where decay cannot happen, such as ice, amber or peat. Fossils show how organisms have changed over time, but the record is incomplete, since soft-bodied creatures rarely fossilise and geological processes destroy much of what does form.
Extinction occurs when the last individual of a species dies. It can be caused by new predators, new diseases, new competitors, a catastrophic event such as an asteroid strike, or environmental change that happens too quickly for the species to adapt to.
Antibiotic resistance: evolution happening quickly
Bacteria reproduce so fast that natural selection can be observed happening over a short time. If a mutation makes one bacterium resistant to an antibiotic, the drug kills off its competitors, the resistant bacterium multiplies freely, and soon the whole infection is resistant, as with MRSA. This process can be slowed by doctors prescribing antibiotics only when needed, patients finishing the full course, and farms restricting antibiotic use in animals.
Classification
Linnaeus classified living things by their visible features into a series of ranks: Kingdom, Phylum, Class, Order, Family, Genus and Species, naming each organism with a two-part binomial made of its genus and species, such as Homo sapiens. As microscopes and biochemistry improved, this system was revised into Woese's three-domain system: Archaea, which are ancient bacteria living in extreme conditions, Bacteria, the true bacteria, and Eukaryota, which includes everything with a nucleus, such as protists, fungi, plants and animals.
Evolutionary trees map how species are related to one another, built using both the characteristics of living species and fossil evidence. The more recently two branches split from a common ancestor, the more closely related those species are.