The problem of genetic variation

Continuous variation
In continuous variation there is a complete range of measurements from one extreme to the other . Height is an example of continuous variation - individuals can have a complete range of heights, for example, 2.00, 2.01 or 2.005 metres high.

Other examples of continuous variation include:

Height
Weight
Hand span
Length of feet
Milk yield in cows

Continuous variation is the combined effect of many genes and is significantly affected by environmental influences. Milk yield in cows, for example, is determined not only by breeding programmes (ie, genetics) but is also significantly affected by environmental factors such as pasture quality and diet, weather, and the comfort of their surroundings.





Discontinuous variation
This is where individuals fall into a number of distinct classes or categories, and is based on features that cannot be measured across a complete range. You either have the characteristic or you don't. Blood group is a good example: you are either one blood group or another - you can't be in between.

Discontinuous variation is controlled by alleles of a single gene or a small number of genes. The environment has little effect on this type of variation.






http://www.bbc.co.uk/schools/gcsebitesize/biology/variationandinheritance/2variationandmutationrev3.shtml