trait

A trait is any feature of an organism you can notice or measure, from eye colour and height to blood type and the way a plant's flowers are shaped. It is the everyday, observable side of biology, the visible result of instructions you cannot see directly.

Each trait arises from an interaction between genes and environment. The genes set up a tendency or a range of possibilities; the environment, including nutrition, light, temperature, and chance events during growth, helps decide where within that range an individual lands. Height, for example, is strongly influenced by genes yet also depends on childhood nutrition.

Traits range from simple to complex. A few are governed mostly by a single gene and fall into clear-cut categories, such as having or not having a certain blood antigen. Most familiar human traits are complex, shaped by many genes and many environmental factors at once, and they vary smoothly across a population rather than splitting into neat types.

It is worth distinguishing a trait, which is observed, from the genotype, which is the underlying genetic recipe. Two individuals can display the same trait for different genetic reasons, and two individuals with similar genes can show different traits if their environments differ.