4.6 Inheritance, variation and evolution — coverage pack

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• It involves one parent and no fusion of gametes.

Method: Award one mark for one parent and one for no gamete fusion; mitosis only or genetically identical offspring are also valid features.

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• Sexual reproduction fuses male and female gametes made by meiosis and mixes genetic information, producing varied offspring. Asexual reproduction uses one parent and mitosis with no gamete fusion, producing genetically identical clones.

Method: Give paired comparisons: gamete fusion versus none, meiosis versus mitosis, two parental gametes versus one parent, and varied versus genetically identical offspring.

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• Seed production is sexual: meiosis forms gametes, pollen and egg nuclei fuse, and genetic information mixes, so the offspring vary. Runner production is asexual: one parent produces new plants by mitosis without gamete fusion, so they are clones.

Method: Identify seeds as sexual and runners as asexual, then link meiosis and fertilisation to mixed genetic information and link mitosis without fertilisation to genetically identical offspring.

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• Meiosis produces four gametes.

Method: Recall that a reproductive cell divides twice during meiosis, giving four products.

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• Meiosis halves the chromosome number to make gametes with one set. At fertilisation, two gametes fuse, restoring the normal number with two sets of chromosomes.

Method: Follow chromosome number rather than naming meiosis stages: halving occurs during gamete formation and fusion restores the full number.

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• Genetic information is copied and meiosis divides the cell twice to make four genetically different gametes with single chromosome sets. Two gametes fuse at fertilisation, restoring the normal number. The fertilised cell divides repeatedly by mitosis, increasing cell number, and cells then differentiate.

Method: Sequence six ideas: DNA copied, two meiotic divisions, four non-identical haploid gametes, fertilisation, mitotic growth and differentiation.

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• Sexual reproduction produces variation; asexual reproduction needs only one parent.

Method: Give one distinct benefit for each method. Other valid asexual benefits include speed, energy efficiency and many offspring in favourable conditions.

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• Asexual reproduction is fast, needs no mate and produces many successful clones in favourable conditions. Sexual reproduction produces genetic variation, so some offspring may survive a change in conditions and be selected.

Method: Link the stable environment to efficient cloning, then link sexual reproduction to variation and a survival advantage under environmental change.

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• Asexual propagation rapidly produces many identical plants that retain the resistant characteristic and needs one parent. However, low genetic variation could make the whole crop vulnerable to a new disease or environmental change. Sexual breeding is slower and may not preserve the exact combination, but variation supplies material for selection and future adaptation. The choice depends on how stable the conditions and disease threats are.

Method: Balance rapid retention of a useful genotype against clone vulnerability, then balance the cost of sexual reproduction against the protective value of variation before giving a conditional judgement.

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• The genome is the entire genetic material of the organism.

Method: The definition must refer to all, or the entire set, of the organism's genetic material.

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• Chromosomes contain DNA. A gene is a small section of DNA on a chromosome. Its base information codes for a particular sequence of amino acids, which forms a specific protein.

Method: Build the hierarchy from chromosome to DNA to gene, then connect the gene's code to amino-acid order and a protein.

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• Genome study can identify genes linked to diseases, support understanding and treatment of inherited disorders, and help trace past human migration. A disease association does not guarantee that a person will develop the disease because other genes or environmental factors may also influence phenotype.

Method: Credit the three specification uses separately. The limitation should distinguish a gene link from certainty, using interaction with other genetic or environmental factors.

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• A sugar, a phosphate group and one base.

Method: List the common sugar and phosphate components and the variable base component.

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• A DNA strand is a polymer of repeating nucleotides with alternating sugar and phosphate sections. Each sugar has one of the bases A, C, G or T attached, and different base orders carry different information.

Method: Describe the repeating nucleotide polymer, the sugar-phosphate arrangement, attachment of one of four bases and variation in base sequence.

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• Each sequence of three bases codes for a particular amino acid. Changing the base order may change a three-base code, so a different amino acid may be placed in the chain. This changes the amino-acid sequence and may alter the protein's structure or function, although some mutations have no effect on phenotype.

Method: Trace the causal chain from altered base sequence to a possible altered triplet, amino acid, amino-acid sequence and protein; retain 'may' because not every mutation changes phenotype.

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• An allele is a form of a gene; heterozygous means having two different alleles for that gene.

Method: Give the precise gene-level definition for allele and contrast the two alleles in a heterozygous genotype.

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• BB and Bb mice have black fur; bb mice have white fur because the recessive phenotype is expressed only with two recessive alleles.

Method: Apply dominance to each genotype: any genotype containing B expresses black, while bb contains no dominant allele and expresses white.

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• Gametes are B and b from each parent; offspring genotypes are BB, Bb, Bb and bb; genotype ratio is 1 BB : 2 Bb : 1 bb; phenotype ratio is 3 black : 1 white; probability of white is $1/4$.

Method: Place B and b gametes from each parent around a Punnett square, combine them in four boxes, count genotypes, then use dominance to count phenotypes. One of four boxes is bb, so the probability is $1/4$.

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• Polydactyly is dominant; cystic fibrosis is recessive.

Method: Match each named disorder to the inheritance pattern specified by AQA.

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• Each parent is heterozygous and has an unaffected dominant allele, so neither shows the disorder. Each can pass on the recessive allele. A child inheriting the recessive allele from both parents is homozygous recessive and has cystic fibrosis.

Method: Explain the parents' carrier phenotypes, inheritance of one allele from each and expression only in the homozygous recessive child.

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• Screening may allow selection of embryos without the disorder, reducing suffering and future medical costs and helping parents make informed choices. It is expensive, may give uncertain or erroneous results, and unused embryos may be rejected or destroyed. Some people object on ethical or religious grounds or fear selection for non-medical traits. A judgement depends on disorder severity, test reliability and the family's values.

Method: Develop at least two benefits and two economic, social or ethical concerns, then make a reasoned conclusion tied to the information rather than asserting that screening is always right or wrong.

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• Females are XX and males are XY.

Method: Recall the two standard sex-chromosome combinations.

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• The mother produces only X eggs; the father produces X and Y sperm. The combinations are XX and XY in equal numbers, so two of four Punnett-square outcomes are XX and the probability is $1/2$.

Method: Cross maternal gametes X and X with paternal gametes X and Y. The four boxes are XX, XY, XX and XY, giving two XX outcomes out of four.

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• The claim is incorrect. Every egg carries X, while a fertilising sperm can carry X or Y. Each fertilisation is independent, so previous outcomes do not alter the next cross and the probability of XX remains $1/2$.

Method: Identify the sex chromosome in each gamete, derive the equal XX and XY outcomes, and apply independence rather than trying to balance the previous births.

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• Genetic causes, environmental causes, and interaction between genes and the environment.

Method: List inherited differences, conditions during development, and their combined effect.

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• The cuttings have the same inherited genes, but environmental conditions such as light, water, mineral availability or temperature differ. The genome interacts with these conditions during development, producing different phenotypes.

Method: Recognise the cuttings as clones, identify relevant environmental differences and link gene-environment interaction to height phenotype.

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• Mutations arise continuously and at random before or during exposure. A rare mutation may give a phenotype that survives the pesticide. Those insects reproduce and pass the variant allele to offspring, so its frequency rises under selection. The pesticide selects existing variation rather than directing insects to mutate.

Method: Separate mutation from selection: random variation appears first, the changed environment creates differential survival, inheritance and reproduction then change allele frequency.

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• Evolution is a change in the inherited characteristics of a population over time.

Method: Include inherited characteristics, a population and change over time.

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• Individuals vary genetically. Those with a favourable phenotype are more likely to survive and reproduce in that environment. They pass the advantageous allele to offspring, so over generations the allele and characteristic become more common.

Method: Use the sequence variation, differential survival, successful reproduction, inheritance and increasing frequency over generations.

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• Speciation is shown if members of the two populations can no longer interbreed to produce fertile offspring. Different environments select different inherited variants; better-suited individuals survive and reproduce, pass on their alleles and make them more frequent. Accumulated inherited differences can eventually produce reproductive isolation.

Method: Do not use appearance alone as the species test. State the fertile-offspring criterion, then trace divergent selection in each environment to accumulated inherited differences and loss of successful interbreeding.

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• Humans breed plants or animals for particular desired genetic characteristics.

Method: Include deliberate human choice and a desired inherited characteristic.

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• Choose resistant parents from a mixed population and breed them. Select resistant offspring and breed those together. Repeat this selection and breeding over many generations until most or all offspring show resistance.

Method: State parent selection, breeding, offspring selection and repetition across generations, keeping the target characteristic genetic.

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• Milk yield may increase over generations and improve food production. However, using a small selected group can cause inbreeding and reduce genetic variation, making inherited defects or disease susceptibility more common. The programme should also monitor health and maintain genetic diversity rather than selecting yield alone.

Method: Explain the intended benefit, connect repeated narrow selection to inbreeding and reduced variation, identify biological consequences and give a balanced judgement.

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• It is modification of an organism's genome by introducing a gene from another organism to give a desired characteristic.

Method: Include genome modification, transfer of a gene from another organism and the desired characteristic.

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• The crop may suffer less damage, need less insecticide and produce a higher yield. However, the modified gene or farming practice may affect wild plants and insect populations, resistant pests could be selected, and some people have health or ethical concerns. Use should depend on evidence from testing and monitoring.

Method: Develop agricultural benefits and ecological or social risks, then make a conclusion based on evidence rather than treating GM crops as automatically safe or harmful.

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• Use enzymes to isolate the required human gene. Insert the gene into a vector, usually a bacterial plasmid or a virus. Use the vector to insert the gene into bacterial cells at an early stage, then grow the modified cells so they express the desired characteristic and reproduce.

Method: Give the ordered stages: isolate with enzymes, place in a vector, transfer into the required cells early in development, then allow the engineered cells to develop and multiply.

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• It can preserve rare plant species or produce plants commercially; the new plants contain the same genetic information because they grow from cells of one parent plant.

Method: Give one specified use and connect asexual growth from one source plant to genetic identity.

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• Allow an animal embryo to begin developing, split its cells before they specialise, form identical embryos and transplant them into host mothers to continue development.

Method: The key order is early embryo, separation before specialisation, identical embryos and transfer to host mothers.

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• Remove the egg nucleus and insert a nucleus from an adult body cell. Apply an electric shock so the cell divides into an embryo. When it is a ball of cells, implant it into a host mother's womb for development. Most genetic information comes from the adult nucleus donor, not the egg or host mother.

Method: Sequence enucleation, nuclear transfer, electric stimulation, embryo division and implantation, then trace the nuclear DNA to the adult body-cell donor.

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• It challenged ideas that God created all species, evidence was limited, and inheritance and variation were not understood.

Method: Select any two of the three reasons specified by AQA.

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• Individuals in a species show variation. Those with characteristics better suited to the environment are more likely to survive and breed. They pass the advantageous characteristics to offspring, so these become more common over generations.

Method: Use the linked sequence of variation, environmental advantage, differential survival and reproduction, inheritance and population change.

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• Darwin proposed pre-existing variation and natural selection: better-suited individuals reproduce and pass inherited characteristics to offspring. Lamarck proposed that changes acquired through use or during life were inherited. Genetic inheritance, fossils and other accumulating evidence support Darwin's mechanism, whereas acquired characteristics are not inherited in the vast majority of cases.

Method: Contrast selection of inherited variation with inheritance of acquired changes, then connect later evidence and understanding of genes to acceptance of Darwin's explanation.

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• He independently proposed evolution by natural selection, gathered evidence, or pioneered work on speciation or warning colouration.

Method: Give one contribution explicitly associated with Wallace in the specification.

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• Wallace independently developed natural selection from worldwide evidence. He and Darwin published joint writings in $1858$, prompting Darwin to publish On the Origin of Species in $1859$. Wallace's work on warning colouration and speciation added evidence and ideas that influenced biology.

Method: Identify independent development, joint publication, its effect on Darwin's publication and Wallace's evidence or specialist contribution.

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• Isolation prevents gene flow between the populations. Mutation and existing genetic variation provide different variants. Different environments favour different phenotypes, so natural selection changes allele frequencies differently. Inherited differences accumulate over many generations until the populations can no longer interbreed to produce fertile offspring, forming new species.

Method: Follow isolation through variation, different selection pressures, differential reproduction, inherited divergence and finally reproductive isolation using the fertile-offspring criterion.

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• Characteristics are determined by inherited units that are passed to descendants unchanged.

Method: Use Mendel's historical 'units' idea and their unchanged transmission between generations.

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• Scientists observed chromosome behaviour during cell division. Chromosomes and Mendel's inherited units behaved in similar ways, leading to the idea that the units, now called genes, are located on chromosomes.

Method: Link the later cell-division observation to similar inheritance behaviour and then to the chromosome location hypothesis for genes.

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• Mendel proposed inherited units from plant crosses in the mid-$19$th century, but their basis was unknown. Later chromosome observations showed similar behaviour, suggesting that genes lie on chromosomes. In the mid-$20$th century DNA structure and gene function were worked out. Evidence and explanations from many scientists accumulated and linked together, producing gene theory.

Method: Give the events in chronological order and explain that later technologies, observations and contributions supplied evidence that connected and refined earlier ideas.

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• Fossils and the evolution of antibiotic resistance in bacteria.

Method: Name the two evidence sources required explicitly by this specification point.

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• Genetics showed that characteristics are passed from parents to offspring in genes. This supplied a mechanism for inheritance of advantageous variation, which was unknown when Darwin published his theory.

Method: Connect genes to inherited characteristics and explain why that connection fills a historical gap in natural selection.

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• Fossils record long-term changes and relationships among organisms through geological time. Antibiotic resistance shows natural selection occurring over short timescales in living bacterial populations. Both fit inheritance through genes, so independent historical and observable evidence converge on the same evolutionary explanation.

Method: Distinguish the timescale and nature of each source, show how each supports population change, and explain that agreement between independent evidence increases confidence.

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• Decay may be prevented, organism parts may be replaced by minerals, or a trace such as a footprint may be preserved.

Method: Give any two distinct formation routes from the specification.

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• Many early organisms were soft-bodied, so they decayed without leaving fossils. Fossilisation requires particular conditions, and geological activity has destroyed many traces. Therefore much evidence is missing and scientists cannot reconstruct the beginning of life with certainty.

Method: Link soft bodies and rare preservation to few original fossils, add destruction by geology, and conclude that gaps limit certainty.

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• P and Q are inferred to be more closely related and to share a more recent common ancestor than either does with R. The tree is a scientific interpretation based on current classification and fossil data; a newly discovered fossil or improved evidence about structures or chemistry could change the proposed relationships.

Method: Read relatedness from the most recent shared branch point, then recognise the tree as an evidence-based model that can be revised when data improve.

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• Extinction occurs when no living individuals of a species remain.

Method: The definition must cover the entire species, not one habitat or population.

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• Examples include environmental change, habitat loss, a new predator, a new competitor, a new disease, a catastrophic event or failure to reproduce.

Method: Give three biologically distinct pressures capable of reducing survival or reproduction until no individuals remain.

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• Habitat loss reduces food, shelter and breeding sites, so population size falls. Predation increases deaths and disease lowers survival or reproduction. A small population contains fewer potential mates and may have low genetic variation, so it is less likely to contain variants suited to rapid change. The interacting pressures may reduce the population to zero.

Method: Link each factor to survival or reproduction, explain why small population size limits recovery or adaptation, and connect the combined effects to no remaining individuals.

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• Bacteria reproduce rapidly and mutations can produce resistant strains.

Method: Link short generation time to repeated reproduction and recognise mutation as the source of resistant variants.

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• A mutation has produced some resistant bacteria. The antibiotic kills susceptible bacteria but resistant ones survive. Survivors reproduce and pass on resistance, so the resistant strain forms a larger proportion of the population.

Method: Describe pre-existing variation, selection by the antibiotic, survival, reproduction and inheritance; do not say the antibiotic deliberately makes bacteria resistant.

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• Avoid prescribing antibiotics for viral or non-serious infections so unnecessary selection is reduced. Patients should complete prescribed courses so surviving bacteria are less likely to remain and reproduce. Restrict agricultural use to reduce selection and spread outside medicine. These measures slow resistance but cannot stop mutations, while replacement antibiotics are costly and slow to develop.

Method: For each measure, explain how it reduces antibiotic exposure or survival and reproduction of resistant strains, then recognise the limitation that mutation continues and new-drug development may not keep pace.

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• Genus and species.

Method: A binomial contains the genus name followed by the species name.

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• Improved microscopes revealed more internal structures, and better understanding of biochemical processes and chemical analysis supplied new evidence. Scientists used this evidence to revise relationships and propose new models such as Woese's three-domain system.

Method: Name developments in evidence or technology, state that they revealed previously unavailable similarities and differences, and link this to revised models.

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• Linnaeus grouped organisms by structure and characteristics in the hierarchy kingdom, phylum, class, order, family, genus and species. Woese used chemical evidence to divide life into archaea, bacteria and eukaryota. Evolutionary trees display proposed relatedness using classification data for living organisms and fossils for extinct organisms. All are evidence-based models and can be revised as evidence improves.

Method: Give the basis and organisation of each system, distinguish the three domains from the Linnaean hierarchy, explain the relational purpose of a tree and finish with the effect of new evidence.