Inheritance, Variation & Evolution

• Sexual reproduction fuses male and female gametes; meiosis makes the gametes and their genetic information mixes at fertilisation.
• Asexual reproduction uses one parent, no gamete fusion and mitosis, so offspring are genetically identical clones unless mutation occurs.
• In animals the gametes are sperm and egg cells; in flowering plants they are pollen and egg cells.

The punchline: Connect sexual reproduction to variation and asexual reproduction to clones.

• In reproductive organs, DNA is copied and the cell divides twice to form four genetically different gametes.
• Each gamete has a single chromosome set; fertilisation joins two gametes and restores the normal number.
• The fertilised cell divides by mitosis, the embryo gains cells and those cells later differentiate.
• AQA does not require the named stages of meiosis.

The punchline: Meiosis makes different haploid gametes; mitosis makes more body cells.

• Asexual reproduction needs no mate, is faster and can produce many identical offspring when conditions are favourable.
• Sexual reproduction takes more time and energy but produces variation that may help some offspring survive environmental change.
• Malaria parasites reproduce asexually in humans but sexually in mosquitoes; fungi reproduce asexually by spores and also sexually.
• Plants can make seeds sexually and reproduce asexually: strawberry runners and daffodil bulb division are named AQA examples.

The punchline: Judge the better method from the conditions, not from a memorised winner.

• DNA is a polymer of two strands forming a double helix; long DNA molecules are packaged as chromosomes.
• A gene is a small DNA section whose base sequence codes for a particular amino-acid sequence and protein.
• The genome is an organism's entire genetic material.
• Genome research can help find disease-linked genes, understand inherited disorders and trace past human migration.

The punchline: Write the scale chain: base sequence → gene → DNA → chromosome → genome.

• Separate Biology: repeating nucleotides form DNA—sugar and phosphate alternate along each strand, with A, C, G or T attached; interpret this structure but need not draw it.
• Three bases code for one amino acid, so base order controls amino-acid order in the protein.
• Higher Tier: A pairs with T and C with G; ribosomes assemble amino acids brought by carrier molecules, then the chain folds.
• Higher Tier: a coding-DNA variant can alter protein activity; a non-coding variant can alter gene expression.

The punchline: Trace any mutation through base sequence, protein shape or amount, then phenotype.

• An allele is a version of a gene. A dominant allele is expressed with one copy; a recessive allele needs two copies to be expressed.
• Homozygous means two identical alleles; heterozygous means two different alleles.
• Genotype is the allele combination; phenotype is the expressed characteristic produced at the molecular level.
• Most characteristics involve multiple genes, and many also interact with environmental conditions.

The punchline: Define the symbols before drawing a cross.

• Write one allele from each parent on the outside, then combine one from each to fill every offspring box.
• Count genotype or phenotype boxes and express the outcome as a probability, fraction, percentage or simple ratio.
• Higher Tier questions may require constructing the entire cross from the information given.
• Each fertilisation is a separate event; a 25% probability does not guarantee one affected child in every four births.

The punchline: Show parental genotypes, gametes, offspring genotypes and the requested probability.

• Polydactyly is caused by a dominant allele; cystic fibrosis is caused by a recessive allele affecting cell membranes.
• Embryo screening can identify embryos with particular alleles; transfer decisions involve medical, economic, social and ethical considerations.
• Evaluate from the information supplied: benefit, limitation, risk, cost and differing viewpoints all matter.

The punchline: Separate the genetic probability from the ethical judgement.

• Ordinary human body cells contain 23 chromosome pairs: 22 autosome pairs and one sex-chromosome pair.
• Females usually have XX sex chromosomes and males usually have XY in the AQA model.
• An XX × XY cross predicts a 1:1 ratio of XX to XY offspring, or a probability of one half for each.

The punchline: The sperm cell supplies the X or Y chromosome in this model.

• Variation means differences among individuals of a population; causes can be genetic, environmental or a combination of both.
• All genetic variants ultimately arise from mutations, which occur continuously.
• Most mutations do not affect phenotype, some influence it and very few create a new phenotype.
• A rare new phenotype may spread rapidly if it suits a changed environment and is inherited.

The punchline: Classify the cause of variation, then explain how genotype and environment interact.

• Across generations, natural selection shifts which inherited characteristics are common in a population; that population-level shift is evolution.
• Better-suited phenotypes can increase survival and reproductive success, so their alleles become more common over generations.
• If divergence prevents two populations from breeding successfully with each other and producing fertile offspring, they have formed separate species.
• All living species are understood to have evolved from simple life forms that appeared more than three billion years ago.

The punchline: Use the chain variation → selection → reproduction → inheritance → population change.

• Choose parents with the desired characteristic, breed them, select suitable offspring and repeat for many generations.
• Targets include disease-resistant crops, higher meat or milk yield, gentle animals and unusual flowers.
• Repeated use of related organisms reduces genetic variation and can increase inherited defects or disease vulnerability.

The punchline: State the repeated selection step and the inbreeding risk.

• In genetic engineering, a selected gene from a donor is added to a recipient organism's genome.
• Examples include bacteria producing human insulin and crops engineered for disease resistance, insect resistance, herbicide resistance or improved yield.
• Potential benefits must be weighed against ecological effects, uncertainty, health concerns raised in the evidence and ethical objections.
• Higher Tier: isolate the gene, insert it into a plasmid or virus vector, then transfer it into cells early in development.

The punchline: Name the gene, vector, recipient cell and desired characteristic when the detail is required.

• Plant tissue culture grows many plants from small cell groups; cuttings grow new plants from part of a parent.
• Embryo transplants split unspecialised embryo cells before placing genetically identical embryos into host mothers.
• Adult cell cloning puts an adult body-cell nucleus into an enucleated egg; an electric shock starts division, then the ball of cells is transferred to a womb.
• Benefits in agriculture or medicine must be evaluated alongside low diversity, welfare risks and ethical objections.

The punchline: A clone shares nuclear genetic information; identical conditions and phenotypes are not guaranteed.

• Darwin developed natural selection from observations and evidence; Wallace independently proposed it, leading to joint writings in 1858 and Darwin's 1859 publication.
• Acceptance was gradual: evidence was limited, inheritance was unknown and the theory challenged established ideas; Wallace's warning-colouration and speciation work added evidence.
• Lamarck proposed that changes acquired during life could be inherited; evidence shows this does not happen in the vast majority of cases.
• Mendel's plant crosses suggested inherited units, but their importance was not recognised until after his death; chromosome and DNA work later linked the units to genes.

The punchline: Explain why evidence and a mechanism changed scientific confidence over time.

• Fossils can form when decay conditions are absent, when tissues are replaced by minerals or when traces such as footprints are preserved.
• Early soft-bodied organisms left few fossils and geological activity destroyed many traces, so the record is incomplete and scientists cannot be certain how life began.
• Fossils, inherited genes and observed antibiotic resistance support evolutionary theory; evolutionary trees organise relationship evidence.
• Extinction occurs when no individuals of a species remain; environmental change, new predators, disease, competition or catastrophic events may contribute.

The punchline: Use evidence to support the theory without pretending the fossil record is complete.

• Random mutations produce bacterial variants; an antibiotic kills susceptible bacteria while resistant bacteria survive.
• Survivors reproduce and pass on resistance, so the resistant strain becomes more common and can spread.
• Reduce selection by avoiding inappropriate prescriptions, completing prescribed courses and restricting agricultural use.
• New antibiotics are costly and slow to develop, so resistance can emerge faster than replacement treatments.

The punchline: Mutation happens first; antibiotic exposure selects the resistant variant.

• The Linnaean hierarchy is kingdom, phylum, class, order, family, genus and species; binomial names use genus and species.
• Woese's domains are archaea (primitive bacteria, often in extremes), bacteria (true bacteria) and eukaryota (protists, fungi, plants and animals).
• Evolutionary trees use current classification and fossil data to show proposed relationships among living and extinct organisms.

The punchline: Read branch points as common ancestry, not as one modern species turning into another.