Variation can be due to inheritance, and also to environmental factors such as climate and diet. Different forms of a certain gene are called alleles. Alleles can be recessive - showing up only when there are two of them - or dominant - showing up always. Cystic fibrosis is an inherited disorder caused when an individual inherits two faulty alleles of a certain gene.

A transgenic organism is one that is created by genetic modification (GM), also called genetic engineering. GM organisms can produce useful substances for us.

In vitro fertilisation (IVF) is a process that helps women who have difficulty in getting pregnant normally. It involves fertilising a woman's egg outside her body, then implanting it back. Some people are worried that IVF will lead to 'designer babies'.

Environmental variation

Our genetic make-up, the sum total of our genes, is called a genotype, but the characteristics which show up in our appearance are called a phenotype.

Genetic variation gives rise to differences between individuals that are inherited. For example, our eye colour is inherited from our parents. But our phenotype is also affected by environmental variation such as:

• climate
• diet
• physical accidents
• culture
• lifestyle

Many kinds of variation are influenced by both environmental and genetic factors, because although our genes decide what characteristics we inherit, our environment affects how these inherited characteristics develop. For example:

• a person might inherit a tendency to be tall, but a poor diet during childhood will cause poor growth
• plants may have the potential for strong growth, but if they do not receive sufficient mineral resources from the soil, they may hardly grow at all

Identical twins are a good example of the interaction between inheritance and environment, as such twins are genetically the same. Any differences you may see between them – for example in personality, tastes and particular aptitudes – are due to differences in their experience or environment.