遗传、变异与进化

Sexual reproduction — fusion of male and female gametes (sperm + egg in animals; pollen + egg in flowering plants). Meiosis forms gametes; mixing of genetic information gives variety in offspring. Asexual reproduction — one parent, no fusion, mitosis only: genetically identical offspring (clones 克隆).

Meiosis 减数分裂: in reproductive organs, copies of the genetic information are made first, then the cell divides twice to form four gametes, each with a single set of chromosomes — all genetically different. Fertilisation restores the normal number; the new cell divides by mitosis and cells differentiate as the embryo develops.

(Bio) Sexual vs asexual: sexual gives variation — a survival advantage if the environment changes, and the variation selective breeding uses; asexual needs no mate (time and energy efficient), is faster, and produces many identical offspring when conditions are favourable. Malarial parasites: asexual in the human host, sexual in the mosquito; many fungi: asexual spores + sexual; strawberry plants: sexual seeds + asexual runners; daffodils: bulb division.

DNA and the genome: DNA is a polymer of two strands forming a double helix, held in the nucleus as chromosomes. A gene 基因 is a small section of DNA that codes for a sequence of amino acids → a specific protein. The genome 基因组 is the entire genetic material of the organism. Importance of the human genome: searching for genes linked to disease; understanding and treating inherited disorders; tracing human migration patterns from the past.

(Bio) DNA structure: a polymer of four different nucleotides 核苷酸 — common sugar + phosphate group + one of four bases — A 腺嘌呤, C 胞嘧啶, G 鸟嘌呤, T 胸腺嘧啶; alternating sugar–phosphate backbone; a sequence of three bases codes for one amino acid, and the base order controls the amino-acid order of the protein.

(HT Bio) Protein synthesis: on ribosomes, from a template; carrier molecules bring specific amino acids in the correct order; the chain folds into a unique shape that lets the protein work as an enzyme, hormone or structural protein (e.g. collagen). In complementary strands C pairs with G, T with A. Mutations change the base sequence: a mutation may leave the protein unchanged, alter it slightly, or change its shape so an enzyme no longer fits its substrate or a structural protein loses strength. Variants in non-coding DNA can switch genes on and off, changing how genes are expressed.

Apply the reproduction and DNA knowledge

Try first — strawberries. Seeds form after fertilisation; runners form without gamete fusion. Identify the two routes and explain which may help a population survive a new disease.

Worked reasoning. Seeds result from sexual reproduction: meiosis makes gametes and fertilisation mixes genetic information. Variation means some offspring may resist the disease. Runners form by mitosis from one parent: clones, apart from mutation. Asexual reproduction needs no mate and is fast in favourable conditions; neither route guarantees survival.

Try first — chromosome number. A human cell has 46 chromosomes before meiosis. Predict the number of gametes, their chromosome number, the number after fertilisation and the division growing the embryo.

Worked reasoning. DNA is copied before two divisions form four gametes, each with 23 chromosomes (one set). DNA copying does not change the count to 92 chromosomes at that point. Fertilisation combines 23 + 23 = 46; mitosis increases embryo cell number and cells differentiate. The diagram models chromosome numbers, not the stages of meiosis.

Try first — scale and code. Distinguish a gene, chromosome and genome. (HT Bio) Write the bases complementary to A C G T and explain whether a mutation must stop an enzyme working.

Worked reasoning. A gene is a DNA section coding an amino-acid sequence for a protein; a chromosome contains a long DNA molecule and many genes; a genome is all the organism’s genetic material. Complementary bases are T G C A. At ribosomes, a template and carrier molecules determine the amino-acid order; the chain folds. A mutation may leave the protein unchanged or change its shape and function. Non-coding variants can change whether genes are expressed.

Learn the terms: gamete 配子 (sex cell), chromosome 染色体 (DNA structure), gene (section coding a protein), allele 等位基因 (a form of a gene), dominant 显性 (expressed with one copy), recessive 隐性 (expressed only with two), homozygous 纯合 (two same alleles), heterozygous 杂合 (two different), genotype 基因型 (alleles present), phenotype 表现型 (characteristics expressed).

Single-gene examples: fur colour in mice; red-green colour blindness. Most characteristics result from multiple genes interacting.

Punnett squares and crosses: complete a Punnett square, extract ratios (e.g. 3:1, 1:1) and probabilities from genetic crosses and family trees. (HT) Construct a cross from parent genotypes and predict outcomes with probability.

Inherited disorders: polydactyly 多指症 (extra fingers or toes) — dominant allele; cystic fibrosis 囊性纤维化 (a disorder of cell membranes) — recessive allele. Make informed judgements about the economic, social and ethical issues of embryo screening.

Sex determination: body cells have 23 pairs of chromosomes; 22 pairs control characteristics, one pair carries the sex genes — female XX, male XY. A sex cross (X×X, X×Y) gives a 1:1 ratio, 50 % chance each.

From terms to family inference and probabilities

Try first — terminology. D causes dominant polydactyly; d does not. Classify Dd and predict its phenotype and gamete alleles.

Worked reasoning. Dd is heterozygous (two different alleles); DD and dd are homozygous. Genotype means the alleles present; phenotype means the characteristics expressed. Dd is affected because one D is expressed. “Dominant” does not mean stronger or more common. Gametes contain D or d, not Dd.

Guided family inference — adapted from AQA June 2024 Paper 2H Q06.2. An affected father and unaffected mother have an unaffected son. Deduce the father’s genotype before reading the answer.

Worked reasoning. The mother and son are dd. The father passed d to his son, but must also carry D because he is affected. Therefore he is Dd. The child’s genotype supplies evidence that the father’s phenotype alone cannot provide.

Construct a cross (HT). Two unaffected CF carriers are Cc and Cc. Each produces gametes C or c. Combining them gives CC, Cc, Cc, cc: probability of CF = 1/4; probability of being a carrier = 1/2. Carriers do not have CF in this recessive model. The separate XX × XY cross gives XX, XX, XY, XY: probability 1/2 each in the GCSE sex-determination model.

Exam transfer — adapted from AQA June 2024 Paper 2H Q06.3. An affected mother Dd and unaffected father dd have three unaffected sons. Construct a cross and predict whether the fourth child will be affected.

Worked reasoning. Egg alleles D/d combine with sperm alleles d/d to give Dd, dd, Dd, dd. Two of four equally likely combinations are affected: probability = 2/4 = 1/2. Earlier births do not change this probability. The four boxes describe possible combinations, not four promised children.

Evaluate — teacher-written screening scenario. Screening can identify embryos with the CF genotype, but treatment is costly and some embryos may not be used. A biological benefit is identifying cc embryos to inform the couple’s decisions. Cost can limit access and creates funding choices. Some families object to selecting or not using embryos; others prioritise reducing inherited disease. An informed judgement uses the stated evidence and recognises different values.

Variation 变异 — differences between individuals in a population — comes from genes inherited (genetic), conditions of development (environmental), or a combination. There is usually extensive genetic variation within a population; all genetic variants arise from mutations — most have no effect on the phenotype, some influence it, very few determine it. A rare mutation giving a new phenotype suited to an environmental change can change the species rapidly.

Evolution: a change in the inherited characteristics of a population over time, through natural selection 自然选择, which may result in a new species. All species evolved from simple life forms that first developed over three billion years ago. Natural selection: variation → the phenotype best suited to the environment survives and breeds → those characteristics are passed on. If two populations become so different they cannot interbreed to produce fertile offspring, two new species have formed.

Selective breeding (artificial selection): choose parents with the desired characteristic from a mixed population → breed → choose the best offspring → repeat over many generations. Uses: disease resistance in food crops; animals with more meat or milk; dogs with gentle natures; large or unusual flowers. Risk: inbreeding 近交 — breeds prone to disease or inherited defects.

Genetic engineering: modifying the genome by introducing a gene from another organism to give a desired characteristic — GM crops resistant to insect attack or herbicides (increased yields); bacteria engineered to make human insulin. (HT) Steps: enzymes isolate the required gene → inserted into a vector (bacterial plasmid or virus) → vector inserts the gene into the required cells → transferred early in development so the organism develops with the desired characteristic. Weigh benefits (medicine, agriculture) against risks (wild-flower and insect populations, unexplored health effects) and ethical objections.

(Bio) Cloning: tissue culture — small groups of plant cells grown into identical new plants (preserving rare species; nurseries); cuttings — simple, older gardeners' method; embryo transplants — splitting unspecialised cells of a developing animal embryo into identical embryos placed in host mothers; adult cell cloning — nucleus removed from an unfertilised egg cell → nucleus from an adult body cell inserted → electric shock makes it divide into an embryo → ball of cells placed in the womb.

Apply variation, selection and biotechnology

Start with variation — teacher-written seedling case. Seedlings differ in inherited disease resistance. Genetically identical plants grow to different heights under different light conditions. Identify genetic and environmental variation, then explain what may happen during a disease outbreak.

Worked reasoning. Resistance alleles provide genetic variation; light conditions can cause environmental differences in height. Phenotype often depends on both. Resistant plants may survive the disease and reproduce more successfully, passing favourable alleles to offspring. Over generations, resistance alleles can become more common. Mutations change DNA; they do not occur because an organism needs a particular change.

Exam transfer — adapted from AQA June 2024 Paper 2H Q01.2. Describe natural selection using insects that already vary in inherited insecticide resistance.

Worked reasoning. The insecticide is a selection pressure: more susceptible insects die. Resistant survivors reproduce and pass resistance alleles to offspring. The population’s inherited characteristics change across generations; an individual does not evolve just by being sprayed. Speciation requires populations to become unable to interbreed to produce fertile offspring.

Compare mechanisms — teacher-written crop case. A grower wants inherited disease resistance without transferring a gene. Select resistant parents from a mixed population, breed them, select resistant offspring and repeat over many generations. This is selective breeding, not natural selection or genetic engineering. Repeatedly breeding close relatives risks inherited defects and reduced genetic variation.

Linked exam explanation — adapted from AQA June 2024 Paper 2H Q09.1. GM soya plants resist glyphosate. Explain how spraying the field can increase yield.

Worked reasoning. Glyphosate kills weeds but the resistant crop survives. Less competition gives the crop more light, water and mineral ions. More light and water can support photosynthesis, producing glucose for respiration and building biomass. Nitrate ions support amino-acid and protein synthesis. Link resources to growth and harvested yield; “GM means higher yield” omits the mechanism. These are authored explanation points checked against the scheme, not an official model or a guaranteed score.

HT — follow a gene. Enzymes isolate a required gene (for example, human insulin). Insert it into a vector such as a bacterial plasmid; use the vector to introduce it into the required cells. The modified cells express the gene and produce the substance. For a developing animal or plant, introduce the gene early so the organism develops with the desired characteristic.

Evaluate — teacher-written GM scenario. An insect-resistant crop suffers less target-pest damage, but a study reports fewer nearby non-target insects without establishing why. Reduced damage may improve harvest. The insect decline raises a food-web concern; compare modified and unmodified fields while accounting for habitat and pesticide use. Association alone does not prove the inserted gene caused the decline. Judge the particular modification and evidence.

Biology — choose and order cloning methods. A nursery can use tissue culture to grow plants from small groups of cells, or cuttings from a parent plant. Embryo transplants split cells before they specialise and place identical embryos into host mothers. Adult cell cloning removes an unfertilised egg’s nucleus, inserts an adult body-cell nucleus, stimulates division with an electric shock, and transfers the ball of cells to a womb. The adult body-cell donor supplies the nuclear genetic information. Environmental differences can still affect a clone’s phenotype.