An organism's phenotype is shaped by its genome and by how the genome interacts with the environment. The genes set what is possible, and the environment decides how far the organism gets there. A plant with genes for tall growth may stay short if it has too little light or too few nutrients. Identical twins have the same genome, but one may become taller than the other because of a better diet.
Some characteristics show discontinuous variation. People fall into distinct groups with no values in between. Eye colour is an example, and it is controlled mainly by genes, with very little effect from the environment.
Other characteristics show continuous variation. There is a whole range of values between the smallest and the largest. Height and weight are examples. They are affected by many genes and also by the environment, such as diet and exercise.
