AQA GCSE Biology 8461: Inheritance, variation and evolution – Study Guide

This guide teaches topic 6, Inheritance, variation and evolution (sections 4.6.1–4.6.4), of the AQA GCSE Biology (8461) specification, for teaching from September 2016 and exams from 2018 onwards. The topic is examined on Paper 2 (1 hour 45 minutes, 100 marks, 50% of the GCSE), set at Foundation and Higher tier. Content the specification marks “(HT only)” is labelled Higher tier only; everything else is for both tiers.

The AQA GCSE Biology hub lists every topic, and the printable checklist lets you tick off each statement.

What this topic covers

Spec section	Content	Higher tier only parts
4.6.1.1–3	Sexual and asexual reproduction, meiosis	–
4.6.1.4–5	DNA, genome, nucleotides	Protein synthesis, mutations, complementary bases
4.6.1.6	Genetic terms, Punnett squares, family trees	Constructing crosses and using probability
4.6.1.7–8	Inherited disorders, sex determination	–
4.6.2.1–2	Variation, evolution	–
4.6.2.3–5	Selective breeding, genetic engineering, cloning	Steps of genetic engineering
4.6.3	Darwin, Wallace, Mendel, speciation, fossils, extinction, resistant bacteria	–
4.6.4	Classification	–

Reproduction (4.6.1.1–4.6.1.3)

Sexual reproduction is the fusion of male and female gametes: sperm and egg in animals, pollen and egg in flowering plants. Genetic information mixes, so offspring vary. Gametes are made by meiosis.

Asexual reproduction needs one parent, with no fusion of gametes and no mixing of genetic information. Offspring are genetically identical clones. Only mitosis is involved.

Meiosis

Cells in reproductive organs divide by meiosis. Copies of the genetic information are made; the cell divides twice to form four gametes, each with a single set of chromosomes; all the gametes are genetically different. At fertilisation, gametes join and the normal number is restored (in humans, 23 + 23 = 46). The new cell divides by mitosis, and as the embryo develops its cells differentiate. You do not need the stages of meiosis.

Advantages of each method

Sexual	Asexual
Produces variation	Only one parent needed
Variation gives a survival advantage by natural selection if the environment changes	Saves time and energy – no need to find a mate
Humans can speed up natural selection by selective breeding	Faster; many identical offspring when conditions are favourable

Some organisms use both. Malarial parasites reproduce asexually in the human host but sexually in the mosquito. Many fungi reproduce asexually by spores and sexually to give variation. Many plants make seeds sexually but also reproduce asexually by runners (strawberry) or bulb division (daffodil).

DNA and the genome (4.6.1.4–4.6.1.5)

DNA is a polymer of two strands forming a double helix, contained in chromosomes. A gene is a small section of DNA on a chromosome that codes for a particular sequence of amino acids, to make a specific protein. The genome is the entire genetic material of an organism.

Understanding the human genome matters for: searching for genes linked to different diseases; understanding and treating inherited disorders; and tracing human migration patterns from the past.

DNA is made of repeating nucleotides. Each has a common sugar and a phosphate group, with one of four bases – A, C, G, T – attached to the sugar. The strands have alternating sugar and phosphate sections. A sequence of three bases codes for one amino acid, so the order of bases controls the order of amino acids in a protein.

Higher tier only.

• On the complementary strands, C always pairs with G and T with A.
• Proteins are made on ribosomes according to a template; carrier molecules bring specific amino acids in the correct order. The finished chain folds into a unique shape, which lets it work as an enzyme, hormone or structural protein such as collagen.
• Mutations occur continuously. Most do not alter the protein, or alter it only slightly. A few produce a protein with a different shape: an enzyme may no longer fit its substrate, or a structural protein may lose strength.
• Non-coding DNA can switch genes on and off, so variants there may change how genes are expressed.

Worked example (Higher tier only). One strand reads ATGCCA. Pair each base: A–T, T–A, G–C, C–G, C–G, A–T. The complementary strand is TACGGT. A coding section of 450 bases gives 450 ÷ 3 = 150 amino acids.

Genetic inheritance (4.6.1.6)

Term	Meaning
Gamete	Sex cell with a single set of chromosomes
Chromosome	Long DNA molecule carrying many genes
Gene	Section of DNA coding for a protein
Allele	A different form of the same gene
Dominant	Always expressed, even with one copy
Recessive	Expressed only with two copies (no dominant allele)
Homozygous	Two identical alleles
Heterozygous	Two different alleles
Genotype	The alleles present
Phenotype	The characteristic expressed

Some characteristics are controlled by a single gene – fur colour in mice and red-green colour blindness in humans. Most characteristics result from multiple genes interacting.

Worked example – Punnett square. In mice, black fur (B) is dominant to brown (b). Two heterozygous black mice (Bb × Bb) are crossed.

          B      b
    B     BB     Bb
    b     Bb     bb

Genotypes: 1 BB : 2 Bb : 1 bb. Phenotypes: 3 black : 1 brown. The probability of a brown pup is 1/4 (0.25 or 25%). In a litter of 12 you would expect about 9 black and 3 brown – real numbers vary because each fertilisation is random.

Higher tier only: you must construct the cross yourself and use probability, e.g. the chance that two separate pups are both brown is 1/4 × 1/4 = 1/16.

Inherited disorders and sex (4.6.1.7–4.6.1.8)

• Polydactyly (extra fingers or toes) – caused by a dominant allele.
• Cystic fibrosis (a disorder of cell membranes) – caused by a recessive allele.

Worked example. Two parents are both carriers of cystic fibrosis (Ff × Ff). Offspring: 1 FF : 2 Ff : 1 ff. Probability of a child with cystic fibrosis = 1/4 (25%); probability of a carrier = 1/2. If one parent has polydactyly (Dd) and the other does not (dd), half the children are expected to have it: 50%.

Family trees: two unaffected parents with an affected child shows the allele is recessive and both parents are carriers.

Embryo screening can identify disorders, but raises issues: cost, the risk of the procedure, and ethical concerns about destroying embryos or choosing characteristics. Use any information given in the question.

Sex determination. Human body cells have 23 pairs of chromosomes: 22 pairs control characteristics only; one pair determines sex. Females are XX, males XY. A cross of XX × XY gives 1 XX : 1 XY, so each child has a 50% chance of being male.

Variation and evolution (4.6.2.1–4.6.2.2)

Variation is differences between individuals in a population, due to genes, the environment in which they developed, or both. There is usually extensive genetic variation within a species. All variants arise from mutations: most have no effect on the phenotype, some influence it, and very few determine it. Very rarely a mutation gives a new phenotype; if it suits an environmental change, the species can change relatively rapidly.

Evolution is a change in the inherited characteristics of a population over time through natural selection, which may produce a new species. All species evolved from simple life forms that first developed more than three billion years ago. If two populations become so different that they can no longer interbreed to produce fertile offspring, they are two new species.

Selective breeding, genetic engineering and cloning (4.6.2.3–4.6.2.5)

Selective breeding (artificial selection): choose parents with the desired characteristic; breed them; select the offspring with the characteristic and breed them; repeat over many generations until all offspring show it. Examples: disease resistance in crops, more meat or milk, gentle dogs, large or unusual flowers. Risk: inbreeding, making some breeds prone to disease or inherited defects.

Genetic engineering modifies the genome of an organism by introducing a gene from another organism to give a desired characteristic. Examples: crops resistant to disease, insects or herbicides, or with bigger, better fruits (GM crops generally give higher yields); bacteria producing human insulin. Concerns include effects on wild flowers and insects, and uncertainty about the effects of eating GM crops. Research is exploring genetic modification to overcome some inherited disorders.

Higher tier only – main steps: enzymes isolate the required gene; it is inserted into a vector, usually a bacterial plasmid or a virus; the vector inserts the gene into the required cells; genes are transferred at an early stage of development so the organism develops with the desired characteristic.

Cloning:

• Tissue culture: small groups of plant cells grow into identical plants – useful for rare species and commercial nurseries.
• Cuttings: a simple, older method used by gardeners.
• Embryo transplants: cells are split from an animal embryo before they specialise, and the identical embryos are transplanted into host mothers.
• Adult cell cloning: the nucleus is removed from an unfertilised egg; the nucleus from an adult body cell (such as a skin cell) is inserted; an electric shock makes it divide into an embryo with the same genetic information as the skin cell; the ball of cells is inserted into the womb of an adult female.

Developing the theory (4.6.3)

Darwin proposed evolution by natural selection after a round-the-world expedition, experimentation and discussion, using geology and fossils: individuals vary; those best suited to the environment are more likely to survive and breed; their characteristics pass to the next generation. He published On the Origin of Species in 1859. Acceptance was slow because the theory challenged the idea that God made all living things, there was too little evidence at the time, and the mechanism of inheritance was not known until 50 years later. Lamarck proposed that changes during an organism’s lifetime are inherited; in the vast majority of cases this cannot happen.

Wallace independently proposed the same theory and published joint writings with Darwin in 1858. He is best known for work on warning colouration and his theory of speciation. Speciation steps: a population is split and isolated; each group has genetic variation; different conditions select different variants; over time the groups become so different they cannot interbreed to produce fertile offspring.

Mendel (mid-19th century) bred plants and found characteristics are determined by “units” passed on unchanged. Chromosome behaviour was seen in the late 19th century; in the early 20th century the units (genes) were linked to chromosomes; in the mid-20th century DNA’s structure was worked out. His work was not recognised until after his death, as chromosomes and genes were then unknown.

Evidence for evolution: genes passing characteristics to offspring, fossils and antibiotic resistance. Fossils form from parts that have not decayed because a condition for decay was absent, from parts replaced by minerals, or as traces such as footprints, burrows and rootlet traces. The record is incomplete because early life was soft-bodied and geological activity destroyed many traces.

Extinction means no individuals remain. Contributing factors include new diseases, new predators or competitors, habitat change and catastrophic events.

Resistant bacteria: bacteria reproduce fast, and mutations produce new strains. A resistant strain survives antibiotic treatment and reproduces, so it spreads (MRSA is an example). To slow this: no inappropriate prescribing (e.g. for viral infections), finish the course, and restrict agricultural use. New antibiotics are costly and slow to develop.

Classification (4.6.4)

Linnaeus grouped organisms by structure: kingdom, phylum, class, order, family, genus, species. The binomial name is genus + species. Better microscopes and biochemistry led to Woese’s three-domain system: archaea, bacteria and eukaryota. Evolutionary trees show how organisms are believed to be related, using classification data for living organisms and fossil data for extinct ones.

Common errors

• Saying meiosis makes two cells, or identical cells.
• Writing a Punnett ratio as a probability without converting (3 : 1 means 3/4 and 1/4).
• Calling a heterozygote “half dominant”.
• Mixing up the gene and the allele.
• Saying individuals “adapt” or “evolve” during their lifetime.
• Stating that antibiotics cause the mutation.

Next, condense this with the revision notes, work through the practice questions, or try the free diagnostics.

Official syllabus

AQA GCSE Biology (8461) specification, Version 1.0 (21 April 2016), for teaching from September 2016 and exams from 2018 onwards, AQA – section 4.6, Inheritance, variation and evolution.