2 Like begets like

It is possible to follow a character, such as eye colour or hair colour in humans, that is handed down from generation to generation. Such characters are said to be inherited characters (or heritable characters) and are determined by genes. A gene can be considered as a unit of inheritance, which determines a particular character and which is passed on from parent to offspring.

Genes maintain the differences between species, such as oak and human, but they also contribute to differences between individuals within a species. For example, consider hair colour or eye colour within a family. Brothers and sisters may share features such as brown hair, that they also share with their biological parents, but in addition they have their own particular combination of characters that make them recognisable as individuals. For example, one sister may have blue eyes whilst her siblings have brown eyes; a brother may have curly hair whilst his siblings have straight hair, and so on. To understand the differences and similarities in characters between individuals, we need to look at how copies of genes are transmitted from parent to offspring. In so doing, we shall discover the rules that govern inheritance.

While at one level there is continuity from one generation to the next, at another level a degree of variation occurs. In fact there is so much variation that every human alive today is different from all others - we are all genetically unique (with the exception of identical twins, who have identical genes). Some of this variation can be seen with the unaided eye, whereas other variation, such as blood groups or the activity of a particular enzyme, is revealed only by more sophisticated molecular biological techniques. The sum of all the characters that an individual organism possesses, not only structural features but also biochemical, behavioural and physiological features, is described as the phenotype. All aspects of an organism's phenotype depend ultimately on that organism's chemical composition and on the biochemical reactions that go on inside it.

The full complement of an individual's genes is called the genotype. The phenotype of each individual is the result of the combined action of their genes (their genotype) and their environment, some characters being influenced more by the environment than are others.

Phenotype, as well as meaning the sum total of all an individual's characters, also has a more restricted meaning; it is used as a shorthand way of referring to the expression of just one character, for example, 'blue-eyed phenotype'. Similarly, genotype is also used to refer to the specific genes associated with a particular character, for example, 'blue-eyed genotype'.