Continous variation occurs when there is a gradation between one extreme and the other of some given characteristic- all individuals exhibit the characteristic but to differing extents.
If a frequency distribution is plotted for such a characteristic a normal distribution is obtained.
Typical examples are: Height, mass, handspan or number of leaves on a plant.
Characteristics which show continous variation are controlled by the combined effect of a number of genes, called polygenes (and are therefore polygenic characteristics) or by the combined eddects of genes and the environment.
Phenotype = genotype + effects of environment.
Discontinous variation occurs when a characteristic is either present or absent (the two extremes) and there are no intermediate forms.
Such variations do not give normal distribution curves but bar charts are often used to illustrate the distribution of a particular characteristic in a population.
Examples are human blood groups in the ABO system (O,A,B or AB), pea plants may be tall or dwarf and tongue-rolling (can or cannot).
- A characteristic which shows discontinous variation is normally controlled by a single gene - there may be two or more alleles of this gene.
- However, if there is more than one gene involved, the gene interact in a epistatic way where one gene masks or influences the expression of another gene.