For temperament, one has the additional problem of how to define it for the purposes of any study. If one speaks of just "high" and "low" temperament with nothing in between, then one would need to show that this has a genetic basis with appropriate testing. More likely, one would find a scale of temperament, just as one would find with loudness of voice, strength of grip or response to sudden noise. Complex traits are likely to be polygenic, such as height, hair color, eye color and so on. When a trait is polygenic it will in general not breed true. With eye color, even two blue-eyed parents can in rare instances have a brown-eyed child.
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