The genome is the entire genetic material of an organism. This means all of the genes encoded in all of the DNA in an organism. The genome interacts with the environment to give some characteristics of the organism, for example, their weight. The various alleles an organism inherits and the interaction of the organism's genome with the organism's environment leads to a large amount of variation between the individuals in a population.
Human blood group is an example of discontinuous variation. There are only 4 types of blood group. There are no other possibilities and there are no values in between. So this is discontinuous variation.
A characteristic of any species with only a limited number of possible values shows discontinuous variation. Here are some examples:
Human height is an example of continuous variation. Height ranges from that of the shortest person in the world to that of the tallest person. Any height is possible between these values. So this is continuous variation.
For any species a characteristic that changes gradually over a range of values shows continuous variation. Examples of such characteristics are:
If you record the heights of a group of people and draw a graph of your results, it usually looks something like this:
The more people measured and the smaller the categories used, the closer the results will be to the curved line. This shape of graph is typical of a feature with continuous variation. Weight and foot length would give graphs similar in shape to this.
Genetic and environmental variation combine together to produce different phenotypes. Discontinuous variation is usually caused by one gene and is not affected by the environment. Your eye colour is the same now as it was when you were younger.
Continuous variation, however, is often caused by a mixture of your genes and your environment. For example, you may inherit the genes to become really tall, but if you don't eat nutritious food then you won't be able to grow as tall as you might.