Trait
A trait is a specific characteristic of an organism, such as eye color or flower shape, that is determined by genes, the environment, or both.
Definition
A trait is a feature of a living thing you can notice or measure, such as eye color, hair texture, or a pea plant's stem height, and it can be inherited through genes, acquired from the environment, or shaped by both. Formally, an inherited trait's genetic makeup is its genotype and its observable form is its phenotype; traits that come in a few sharp categories, such as Mendel's seven pea traits, usually depend on one or two genes, while traits like height or weight that vary smoothly usually depend on many genes plus environment. At the most general level, a trait (or character) ranges from a discrete Mendelian character controlled by a single locus to a polygenic quantitative trait shaped by many loci of small effect, and its heritability is the fraction of its variance in a population attributable to genetic variance, estimable from twin, family, or genome-wide association studies. Traits can also show pleiotropy, epistasis, and gene-by-environment interaction, so the same gene can influence several traits at once.
Example
Attached earlobes and dimples are inherited human traits, while a suntan is not inherited even though the ability to tan is; human blood type is controlled by one gene with three alleles, but skin color depends on many genes plus sun exposure and so varies continuously. Adult height in well-nourished populations has a heritability near $0.8$, yet thousands of identified genetic variants each shift height by only about a millimeter, while sickle-cell versus normal hemoglobin depends on a single base change in one gene.
Key Insight
Heritability describes variation across a population, not the cause of a trait in one person, and it shifts with the environment, which is why average height rose sharply during the twentieth century without any genetic change at all.