Variation within populations

1. Individuals of the same species share many features, but a population usually contains a wide range of inherited differences.
2. Members of a population usually have the same kinds of genes, while the alleles they carry and the combinations of those alleles can differ.
3. A gene can have several alleles across a population, even though a diploid individual normally carries only two copies of that gene, one inherited from each parent.
4. Genetic variation therefore includes both differences between DNA base sequences and differences in the combinations of alleles present in individuals.
5. Two organisms can look very similar while differing at many positions in their DNA, so visible appearance alone underestimates genetic variation.
6. The illustration shows different bases at the same position in a DNA sequence, which is one possible form of genetic variation.

Sources of genetic differences

1. Mutations are the original source of new genetic variants, including new alleles.
2. They occur without being directed towards a particular useful outcome, so an organism does not produce a specific mutation because it needs that characteristic.
3. A mutation can be passed to offspring only if the altered genetic material is included in the cells that produce the offspring.
4. In sexually reproducing animals, a mutation confined to an ordinary body cell is generally not passed to the next generation.
5. Sexual reproduction brings existing alleles together in new combinations, which explains why offspring usually differ from both their parents and their siblings.
6. Meiosis produces genetically different gametes, and the random meeting of gametes during fertilisation adds further combinations.
7. The chromosome combinations in the diagram illustrate this mixing of inherited material; the mixing rearranges existing alleles rather than creating new versions of them.

Genetic and visible variation

1. Genotype is the combination of alleles an organism has.
2. Phenotype is the observable characteristics of an organism, produced by its genotype and its interactions with the environment.
3. A difference in phenotype can result from genetic differences, environmental differences or both, so visible variation is not automatically evidence of different alleles.
4. For example, a plant may be short because it carries alleles associated with short growth, because it receives too few mineral ions, or because both factors affect it.
5. Most genetic variants do not change the phenotype, while some alter a characteristic and a small number can have a large effect.
6. A variant with no obvious effect is still a DNA difference and therefore still contributes to genetic variation.
7. When comparing individuals, use evidence about their inherited material to distinguish genetic variation from changes caused only by their surroundings.