This topic links the molecule of inheritance (DNA) to the bigger picture of how living things vary, are bred, and evolve. You will study sexual and asexual reproduction, meiosis, how genes control characteristics, and how to predict the outcomes of genetic crosses using Punnett squares. You then move outward from the cell to the population: how variation and mutation provide the raw material for evolution, how natural selection and selective breeding shape organisms, and how scientists use genetic engineering and cloning. The topic finishes with the evidence for evolution, the formation of new species, antibiotic-resistant bacteria, and how we classify life. Higher-tier and biology-only content is flagged where it applies to AQA 8461.
Sexual reproduction involves the fusion of male and female gametes: sperm and egg cells in animals, pollen and egg cells in flowering plants. It mixes genetic information from two parents, so offspring show variation. Asexual reproduction involves only one parent and no fusion of gametes, no mixing of genetic information. It uses mitosis, so offspring are genetically identical clones of the parent.
Gametes are produced by meiosis, which happens only in the reproductive organs. Meiosis: a cell copies its DNA, then divides twice to form four gametes, each with a single set of chromosomes (half the number of the parent cell). All gametes are genetically different from each other. When two gametes fuse at fertilisation, the full chromosome number is restored, and the new cell divides by mitosis to form an embryo, with cells later differentiating.
Some organisms can use both types of reproduction depending on circumstances. Advantages of sexual reproduction: it produces variation in offspring, so if the environment changes variation gives a survival advantage by natural selection; this can be sped up by humans through selective breeding. Advantages of asexual reproduction: only one parent is needed, it is more time- and energy-efficient (no mate required), and it is faster, allowing many identical offspring when conditions are favourable.
Examples that use both: malarial parasites reproduce asexually in the human host but sexually in the mosquito; many fungi reproduce asexually by spores but also sexually; plants such as strawberries and daffodils produce seeds sexually but also reproduce asexually by runners or bulbs.
The genetic material in the nucleus of a cell is composed of a chemical called DNA, a polymer made of two strands forming a double helix. DNA is contained in structures called chromosomes. A gene is a small section of DNA on a chromosome that codes for a particular sequence of amino acids, which fold to make a specific protein. The genome of an organism is the entire genetic material of that organism.
Understanding the human genome is important because it lets scientists search for genes linked to different types of disease, understand and treat inherited disorders, and trace human migration patterns from the past.
DNA is made of nucleotides. Each nucleotide has a common sugar and phosphate group with one of four bases attached: A, C, G and T. A sequence of three bases is the code for a particular amino acid; the order of bases controls the order of amino acids in a protein. Long strands of nucleotides join to form polymers.
In protein synthesis, the gene is not used directly. A copy of the gene (a template molecule, mRNA) carries the code out of the nucleus to ribosomes, where the protein is assembled. Carrier molecules bring specific amino acids in the order coded for; the amino acid chain then folds into a unique shape, which lets proteins such as enzymes, hormones and structural proteins (e.g. collagen) do their jobs. Mutations occur continuously. Most have no effect, some alter the protein's activity. In non-coding DNA, mutations may change how genes are switched on and off and so affect phenotype.
Some characteristics are controlled by a single gene, e.g. fur colour in mice. Genes can have different forms called alleles. The alleles present (the genotype) determine the characteristic shown (the phenotype). If both alleles are the same, the organism is homozygous; if different, heterozygous. A dominant allele is always expressed even if only one copy is present; a recessive allele is only expressed if two copies are present (no dominant allele).
To predict a cross, use a Punnett square. For example, crossing two heterozygous individuals (Bb × Bb) gives offspring in the ratio 1 BB : 2 Bb : 1 bb, so a 3:1 ratio of dominant to recessive phenotype, equivalent to a 75% : 25% (3/4 : 1/4) probability. Most characteristics are controlled by multiple genes interacting, not a single gene.
Some disorders are inherited. Polydactyly (extra fingers or toes) is caused by a dominant allele, so it can be passed on by one affected parent. Cystic fibrosis, a disorder of cell membranes, is caused by a recessive allele, so a child must inherit one recessive allele from each parent; carriers show no symptoms. Embryo screening and gene therapy raise economic, social and ethical issues, which is good context for evaluating the technology.
Ordinary human body cells contain 23 pairs of chromosomes. 22 pairs control characteristics only; one pair carries the genes that determine sex. In females the sex chromosomes are the same (XX); in males they are different (XY). A Punnett cross of XX × XY predicts equal numbers of male and female offspring, a 1:1 (50%) ratio.
Differences in the characteristics of individuals of the same kind are called variation. Variation may be due to differences in genes inherited (genetic causes), the environment in which they have developed (environmental causes), or a combination of both. Usually there is extensive genetic variation within a population.
All variation arises from mutations: random changes in the DNA. Mutations occur continuously. Most have no effect on the phenotype; some influence it; very rarely a single mutation significantly affects the phenotype. New alleles produced by mutation are mostly neutral, but occasionally a mutation leads to a new phenotype. If the new phenotype is suited to an environmental change, it can give a survival advantage and lead to relatively rapid change in a species by natural selection.
Evolution is a change in the inherited characteristics of a population over time through a process of natural selection, which may result in the formation of a new species. The theory states that all species of living things have evolved from simple life forms that first developed more than three billion years ago.
The mechanism, proposed by Charles Darwin, is natural selection. Within a species there is variation. Organisms with characteristics most suited to the environment are more likely to survive to breed successfully. The genes that gave them an advantage are then passed on to their offspring. Over many generations, beneficial alleles become more common. If two populations of one species become so different in phenotype that they can no longer interbreed to produce fertile offspring, they have formed two new species.
Darwin published On the Origin of Species in 1859. His theory of evolution by natural selection was only gradually accepted because it challenged the idea that God made all the animals and plants, there was insufficient evidence at the time to convince many scientists, and the mechanism of inheritance and variation (genes and mutations) was not known until 50 years after the theory was published.
Alfred Russel Wallace independently proposed the theory of evolution by natural selection and published jointly with Darwin in 1858; he is best known for his work on warning colouration in animals and his theory of speciation. Other theories, including that of Jean-Baptiste Lamarck (based on the idea that changes acquired during an organism's lifetime can be inherited), are mostly rejected today.
(Biology only) Mendel carried out breeding experiments on plants in the mid-1800s and concluded that the inheritance of each characteristic is determined by separately inherited 'units'. We now call these genes. The importance of his work was not recognised until after his death, once chromosomes and then DNA were discovered.
Fossils (remains of organisms from millions of years ago, found in rocks) form by gradual replacement of hard parts by minerals, as casts or impressions, or as preserved parts where decay did not happen. The fossil record is incomplete because early soft-bodied organisms left few traces and many fossils have been destroyed by geological activity. Extinction happens when there are no individuals of a species still alive; causes include new predators, new diseases, new competitors, catastrophic events and changes to the environment. Antibiotic-resistant bacteria such as MRSA evolve rapidly because bacteria mutate and reproduce quickly; resistant bacteria survive and pass on resistance. To reduce resistance: doctors should not over-prescribe antibiotics, patients should complete their course, and the agricultural use of antibiotics should be restricted. (Higher: speciation by isolation of two populations leading to different selection pressures.)
Selective breeding (artificial selection) is choosing parents with desired characteristics and breeding them together over many generations to develop a desired feature, e.g. disease resistance in crops, gentle nature in animals, high yield or large flowers. A risk is reduced variation, which can lead to inbreeding where harmful recessive alleles and disease are more likely.
Genetic engineering modifies the genome of an organism by introducing a gene from another organism to give a desired characteristic, e.g. bacteria producing human insulin, or genetically modified (GM) crops resistant to disease or insects, or with bigger yields. (Higher: enzymes cut out the required gene, which is inserted into a vector, usually a bacterial plasmid or virus, then transferred to the target cells at an early stage so all cells develop with the new gene.) There are benefits and concerns about GM crops, including effects on wild flowers, insects and human health.
(Biology only) Cloning methods include: tissue culture, using small groups of plant cells to grow identical plants; cuttings, an older method for identical plants; embryo transplants, splitting an embryo's cells before they specialise and transplanting them into host mothers; and adult cell cloning, where the nucleus of an adult body cell replaces the removed nucleus of an egg cell, which is stimulated to divide and the embryo implanted into a host.
Traditionally living things have been classified into groups depending on their structure and characteristics, in a system developed by Carl Linnaeus. He grouped organisms into kingdom, phylum, class, order, family, genus and species. Organisms are named by the binomial system of genus and species, e.g. Homo sapiens.
As microscopes and biochemical understanding improved, classification changed. Due to evidence from chemical analysis, Carl Woese proposed the three-domain system: Archaea (primitive bacteria, often in extreme places), Bacteria (true bacteria), and Eukaryota (protists, fungi, plants and animals). Evolutionary trees are models that use current classification data for living organisms and fossil data for extinct organisms to show how scientists believe organisms are related.
Practise exam-style questions on this topic.