Sexual and asexual reproduction, meiosis, DNA and the genome, genetic inheritance and inherited disorders, variation and mutation, evolution by natural selection, selective breeding, genetic engineering, and evidence for evolution including fossils and extinction.

What this topic is asking

AQA wants you to compare sexual and asexual reproduction and describe meiosis, explain DNA and the genome, use genetic diagrams and ratios, describe inherited disorders, explain variation and mutation, and explain evolution by natural selection along with selective breeding, genetic engineering and the evidence for evolution.

Reproduction and meiosis

Meiosis happens only in the reproductive organs and produces four genetically different gametes, each with half the number of chromosomes (in humans, 23 instead of 46). It does this through two divisions: the chromosomes are copied, then the cell divides twice so each gamete ends up with a single set. Genetic variation arises because the maternal and paternal chromosomes are shuffled. When two gametes fuse at fertilisation, the full chromosome number (the diploid number) is restored, and the new cell divides by mitosis to form an embryo. Asexual reproduction, by contrast, needs no gametes and produces offspring that are clones, which is fast and reliable but provides no variation to cope with environmental change.

DNA, the genome and inheritance

DNA is a polymer made of two strands coiled into a double helix, stored in structures called chromosomes inside the nucleus. A gene is a small section of DNA that codes for a particular sequence of amino acids, and so for a specific protein, which in turn affects a characteristic. The genome is the entire genetic material of an organism; the Human Genome Project mapped all human genes and helps scientists search for genes linked to disease, understand inherited disorders and trace human migration.

Genetic-cross diagrams (Punnett squares) predict the proportions of offspring genotypes and phenotypes. Inherited disorders include polydactyly (extra fingers or toes, caused by a dominant allele, so only one parent need carry it) and cystic fibrosis (a disorder of cell membranes, caused by a recessive allele, so both parents must be carriers). Sex is determined by the sex chromosomes: XX is female and XY is male, so a cross of XX with XY gives a 1 to 1 ratio of females to males.

Variation and evolution

Variation within a population is caused by differences in genes (inherited), differences in the environment, or a combination of both. Mutations are random changes to the DNA base sequence that occur continuously; most have little or no effect on the phenotype, a few are harmful, and very occasionally one produces a new characteristic that is advantageous, providing the raw material for evolution.

Humans also change organisms deliberately. Selective breeding (artificial selection) chooses parents with a desired characteristic (high milk yield, disease resistance, large fruit) and breeds them over many generations, though it can reduce the gene pool and cause inbreeding problems. Genetic engineering cuts a useful gene from one organism and transfers it into another, for example inserting the human insulin gene into bacteria so they make insulin, or making crops resistant to pests or herbicides; there are benefits (higher yields, medicines) and concerns (effects on wild populations and ethics). Evidence for evolution comes from fossils (showing how organisms changed over time, though the early fossil record is incomplete because soft-bodied organisms rarely fossilise) and from observing rapid evolution such as antibiotic-resistant bacteria. Extinction occurs when the environment changes faster than a species can adapt, a new predator or disease appears, or a catastrophic event happens, so no individuals of the species remain.