> Is there any trait that is strongly linked to a single gene? Sure. Sickle-cell anaemia is one classic case, one (recessive) gene. Huntington's disease is a

> Is there any trait that is strongly linked to a single gene? Sure. Sickle-cell anaemia is one classic case, one (recessive) gene. Huntington's disease is another, it's about a specific repeat number. But, as you say, most complex traits (such as height) aren't like that, and involve hundreds or thousands of different genes. IIRC eye color is actually fairly simple, not one gene but much of the control in just a handful? Maybe hair color too? (But not super-sure.)