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Why is eye colour generally less influenced by ordinary environmental changes than height or weight?

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Four kittens with different fur patterns-three tabby and one black-and-white-lying together in a box, illustrating genetic variation among offspring. — heredity Each offspring is a combination of its two parents, receiving some dominant traits from its mother and others from its father.

Eye colour is generally less influenced by ordinary environmental changes than traits like height or weight because it is primarily under direct hereditary control. Specifically, one or more genes specify the synthesis and deposition of melanin within the iris, making the trait relatively independent of external factors such as diet or climate.1

In contrast, traits like height and weight are significantly more susceptible to environmental influence. While genetics sets the potential for stature, environmental factors—most notably nutrition, health, and hormones—determine how fully that potential is realized. For example, growth can be stunted in the absence of adequate nutrition, whereas a nutrient-dense diet helps the body reach its full potential. Because these traits are polygenic and highly responsive to external conditions, their final expression is the result of a complex nature-nurture interaction that is far more variable than the pigmentation of the eye.123