Single gene inheritance, with one gene and a dominant and a recessive allele, controls only a few features. Tall or dwarf in pea plants and the inherited disorder cystic fibrosis are examples. Each of these gives clear groups with nothing in between.
Most phenotypic features are the result of many genes working together, not a single gene. They are called polygenic features, and in a polygenic feature the phenotype is controlled by multiple genes. Human height and skin colour are examples. Each gene has only a small effect, and there are many possible combinations of alleles, so the phenotypes form a continuous range from one extreme to the other rather than two clear groups. This is continuous variation.
The environment also affects these features. Height, for example, depends on the genes inherited but also on diet during childhood. Because so many genes are involved, a simple Punnett square for one gene cannot predict the phenotype of the offspring.
