Variation

• Variation: differences between individuals of the same species
• Phenotypic variation is caused by both genetic and environmental factors
• Continuous variation is influenced by genes and environment, resulting in a range of phenotypes between two extremes, e.g. height in humans
• Discontinuous variation is caused by genes alone and results in a limited number of distinct phenotypes (e.g. you are either blood group O, A, B or AB, nothing else)
• Mutation: genetic change
• Gene mutation: a change in the base sequence of DNA
• Mutation is the way in which new alleles are formed
• Ionising radiation and some chemicals increase the rate of mutation
• Mutation is a source of variation e.g. in Down’s syndrome, where a parent’s chromosomes are unevenly distributed in meiosis. In fertilisation, a zygote with a number of chromosomes that is not 46 is created (e.g. 23 + 24). Characteristics: broad forehead, short neck, downward-sloping eyes, short nose and mental retardation.