Characteristics are passed on in DNA. Small variations in what is passed on, acted on by natural selection over very long periods, account for the diversity of life.
Sexual reproduction involves the fusion of male and female gametes, mixing genetic information from two parents, so the offspring show variation. Asexual reproduction involves one parent and no fusion of gametes. In many eukaryotic organisms it involves mitosis, so the offspring are genetically identical clones.
Sexual reproduction produces variation, which gives a survival advantage if the environment changes and allows selective breeding to increase food production. Asexual reproduction needs only one parent, uses less energy, is faster, and produces many identical offspring when conditions are favourable.
Some organisms do both: malarial parasites reproduce asexually in the human host and sexually in the mosquito; many fungi and many plants can do either.
Meiosis makes gametes. Cells in the reproductive organs copy their genetic information and then divide twice, giving four gametes, each with a single set of chromosomes, half the usual number.
The gametes produced are usually genetically different from one another. At fertilisation the normal number is restored, and the new cell then divides by mitosis, with the cells differentiating as the embryo develops.
The genetic material in the nucleus is DNA, a polymer wound into a double helix and contained in structures called chromosomes, which occur in homologous pairs in diploid body cells.
A gene is a small section of DNA coding for a particular sequence of amino acids, making a specific protein. The genome is the entire genetic material of an organism.
Understanding the human genome helps in finding genes linked to disease, in understanding and treating inherited disorders, and in tracing human migration patterns.
Genome sequencing has become much faster and cheaper, and is increasingly used in medicine.
It can identify genes linked to particular diseases, so a person’s risk can be estimated before symptoms appear and monitoring or prevention started early. It can help understand inherited disorders, and it can guide treatment: some drugs work well for people with one version of a gene and poorly for others, so sequencing can inform the choice of drug and dose.
It does not predict with certainty. Most conditions depend on many genes together with the environment, so a genome gives a probability rather than a diagnosis.
DNA is made of nucleotides, each a sugar, a phosphate and one of four bases: A, C, G and T. The two strands are held together by complementary base pairing: A with T, C with G.
A sequence of three bases codes for one amino acid. The order of bases therefore controls the order in which amino acids are assembled, and so which protein is made and how it folds. Some non-coding regions of DNA help regulate whether genes are switched on or off.
An allele is a version of a gene. A dominant allele is expressed if only one copy is present; a recessive allele only if both copies are.
Homozygous means the two alleles are the same; heterozygous means they differ. The genotype is the combination of alleles; the phenotype is the characteristic that results.
A Punnett square crosses the parents’ alleles to give the possible offspring genotypes and the expected ratio. It is a prediction of probability, not a guarantee of what any particular offspring will be.
Most characteristics are controlled by several genes interacting, not a single one. Height and skin colour are examples; single-gene characteristics are the exception rather than the rule.
Polydactyly, having extra fingers or toes, is caused by a dominant allele, so inheriting one copy is sufficient for the characteristic to be expressed. Cystic fibrosis, a disorder of cell membranes, is caused by a recessive allele, so a person with the condition has inherited that allele from both parents. If two unaffected carriers have a child, there is a one in four chance that the child will have cystic fibrosis.
Embryos can be screened for such alleles during IVF. Arguments against screening include the risk of discrimination against those with a disorder, the cost, the implication that people with the condition are less valuable, and the possibility of selecting for other characteristics; arguments in favour include reducing suffering and treatment costs.
The ABO blood group system is controlled by three alleles, not two. IA and IB are both dominant over IO, but neither is dominant over the other.
Someone with IAIB therefore has blood group AB, with both alleles expressed. This is codominance. Group O requires two IO alleles.
One of the 23 pairs of human chromosomes carries the genes that determine sex. In the simplified GCSE model, XX usually leads to female development and XY to male development. A Punnett square gives a 50% chance of each.
The X chromosome carries genes the much smaller Y chromosome does not. A male has only one X, so a single recessive allele on it is expressed, with no second copy to mask it. A female would need two copies to be affected, and with one copy she is a carrier. Conditions such as red-green colour blindness and haemophilia are therefore far more common in males.
Differences between individuals of the same species arise from their genes, from their environment, or from both together.
Mutations occur continuously. Most have no effect on the phenotype; some influence it slightly; very rarely one determines it. A mutation in a coding region may change the protein’s shape and so its function; one in a non-coding region may change how a gene is expressed.
Evolution is a change in the inherited characteristics of a population over time, through natural selection, which may result in a new species.
Individuals vary. Those with advantageous inherited characteristics are more likely to survive and reproduce, and to pass those characteristics on. Over many generations the population changes.
Darwin proposed this after observations on the Beagle, wide experimenting and discussion, and knowledge of fossils and geology. It was only gradually accepted, because it challenged the idea that God made all the animals and plants, there was insufficient evidence at the time, and the mechanism of inheritance was not understood for another fifty years.
Lamarck proposed instead that characteristics acquired during an organism's life are inherited, so that a giraffe stretching its neck would pass on a longer neck. That was not supported by later evidence. Wallace worked independently on natural selection and published jointly with Darwin in 1858.
Wallace did much of the early work on speciation: if separated populations become genetically different enough that they can no longer interbreed to produce fertile offspring, they are considered separate species.
Mendel carried out breeding experiments on pea plants in the 1850s and concluded that inherited characteristics are determined by separate ‘units’ passed on unchanged. His work was not recognised in his lifetime. Chromosomes were observed in the late 1800s, their behaviour was linked to Mendel’s units in the early 1900s, the units were identified as genes on chromosomes, and DNA’s structure was determined in 1953.
The theory is now widely accepted because the evidence supports it from several independent directions.
Fossils are the remains of organisms from millions of years ago, formed when parts did not decay because a condition for decay was absent, when parts were replaced by minerals, or as preserved traces such as footprints. Many early organisms were soft-bodied and left little trace, and much of what did form has been destroyed by geological activity, so the record is incomplete, especially for the earliest life, and scientists cannot reconstruct every stage of evolutionary history.
The pentadactyl limb, the same five-digit bone pattern in the limbs of very different vertebrates, is evidence of a common ancestor.
Human evolution is traced from fossils such as Ardi and Lucy, and from the increasing sophistication of stone tools, which can be dated by their position in rock layers.
Antibiotic-resistant bacteria such as MRSA show natural selection happening now: genetic variation means some bacteria may already be resistant, often through mutation; the antibiotic kills the susceptible ones, and the resistant bacteria survive, reproduce and spread.
Extinction means no individuals of a species remain. It happens when the environment changes too fast for the species to adapt, when a new predator or disease arrives, when a competitor out-competes it, or through a catastrophic event such as an asteroid impact.
Selective breeding is choosing the individuals with the desired characteristic and breeding them together, repeating over many generations. It has been done for thousands of years, in food crops, in disease-resistant plants, in domestic animals and in flowers.
The risk is inbreeding: the population becomes closely related, so there is less variation, they are more vulnerable to a new disease, and rare harmful alleles are more likely to appear in a homozygous form.
Genetic engineering transfers selected genetic material into the genome of another organism. Restriction enzymes can be used to cut out the desired gene, and a vector such as a plasmid or a virus carries it into the target cell. In microorganisms the modified cells then reproduce to form a population. In multicellular organisms the genetic material may be introduced at an early stage of development, so that many cells carry the new gene.
Crops have been engineered for insect resistance and better yields, bacteria to produce human insulin, and there is the prospect of treating inherited disorders. Concerns raised include the effect on populations of wild flowers and insects, the view held by some people that the effects of eating GM crops on human health have not been fully explored, and disputes over who controls and profits from the technology.
Cloning produces genetically identical individuals. Tissue culture grows small pieces of plant tissue on a sterile nutrient medium; cuttings are an older, simpler method for plants; embryo transplants split an early embryo and implant the parts in host mothers; and adult cell cloning replaces the nucleus of an unfertilised egg with the nucleus of an adult body cell, stimulates it to divide, and implants the embryo.
Linnaeus classified living things into kingdom, phylum, class, order, family, genus and species, and named them by the binomial system of genus and species.
As microscopes improved and biochemical processes became better understood, new models were proposed. Carl Woese divided life into three domains, archaea, bacteria and eukaryota, on the basis of comparisons of ribosomal RNA sequences.
Evolutionary trees use classification data for living organisms and fossil data for extinct ones to show how closely species are related.
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