Some features are controlled by a single gene. Cystic fibrosis is an example. A single gene with two alleles gives just a few possible phenotypes, so we can predict the outcome with a Punnett square.
However, most phenotypic features are the result of multiple genes working together, rather than single gene inheritance. Human height and skin colour are examples. Each of the many genes adds a small effect, so the offspring can show a wide range of phenotypes, not just two clear types.
Because several genes are involved, a simple monohybrid cross cannot explain these features. Their phenotypes also form a continuous range of values, such as every height between short and tall.
