Continuous variation is when individuals in a population vary within a range with no distinct categories; these can be things like height, mass or skin colour.

Discontinuous variation is when individuals are in a specific category, and will fall into only one category, these can be things like sex, eye colour and blood group.

Some characteristics are controlled by only one gene, and are called monogenic, these tend to show discontinuous variations.

Most traits are polygenic which is when they are controlled by a number of genes are different loci. When one gene is affected by another it is known as epistasis.

Some characteristics are only influenced by genotype such as blood group, but some are influenced by the environment and genotype. Some of these include Height, monoamine oxidase A, Cancer and animal hair colour.


Since 1850 the human population has on average got 8cm taller. This can be due to many reasons such as:

  • Taller men have more children, resulting in a gradual change
  • Greater movements of people have lead to less inbreeding
  • Improved health, especially in protein
  • The end of child labour has allowed more energy to be put into growth.

Hair colour

In animals they make melanin using the enzyme Tyrosinase. This is the first step along a chemical pathway. Some animals have light hair in the summer and dark in the winter this is due to a temperature trigger.


Monoamine oxidase A is an enzyme that catalyses the breakdown of neurotransmitters in the brain involved in the regulation of behaviour. It has been found that in some individuals they have a rare mutation of the MAOA gene, where no enzymes are produced; these people portray a very aggressive behaviour.



It is the uncontrolled division of cells that leads to a lump of cells (tumours) forming. It is made up of both an environmental and genetic threshold.