threshold trait
A threshold trait looks all-or-nothing on the surface — you either have the condition or you don't — yet underneath sits a hidden, continuous scale. Picture a reservoir filling behind a dam: nothing visible changes until the water tops the wall, and then it spills. Many genes and environmental factors raise or lower an unseen quantity called liability, and the trait appears only once that liability crosses a critical line.
This model neatly reconciles two facts that seem at odds: an outcome that is categorical, present or absent, yet clearly polygenic and influenced by environment. The underlying liability is assumed to be normally distributed, like any quantitative trait, and the threshold simply slices that smooth distribution into two visible groups. People just below the line carry nearly as much risk as those just above it, even though only one group shows the trait.
The threshold framework explains several real-world patterns. Relatives of an affected person sit, on average, higher on the liability scale, so their risk is raised; and when the two sexes differ in how easily they cross the threshold, the less frequently affected sex tends to need a heavier genetic load, passing higher risk to their relatives. It is a bridge connecting discrete clinical categories back to continuous quantitative genetics.
Cleft lip is treated as a threshold trait: many small genetic and prenatal factors set a baby's liability, and the cleft forms only when their combined total pushes liability past the developmental threshold.
A categorical outcome from a continuous hidden liability.
The hidden continuous scale is called liability. Because we cannot measure it directly, threshold traits are studied through patterns of risk in relatives rather than by reading liability off any individual.