Some features are controlled by a single gene. For example, cystic fibrosis is an inherited disorder caused by one gene with a recessive allele. These features usually have only two or three clear phenotypes, and they can be worked out with a monohybrid cross.
However, most phenotypic features are the result of multiple genes working together, rather than single gene inheritance. Height, skin colour and eye colour are controlled by more than one gene. Each gene has a small effect, and the effects add together, so many genes are involved.
Because many genes contribute, these features show a wide range of phenotypes, not just two types. This is why people come in many different heights, not simply tall or short. The environment can affect these features as well.
