Which factor influences the colour of eye?

A physical trait present from birth — one whose overall pattern is not altered by a person's surroundings, physiology, or upbringing — points to a biological cause rather than an environmental or social one. Such inborn characteristics are transmitted from parents to offspring through genetic information carried in genes and chromosomes; this mode of transmission is heredity.

Eye colour is exactly this kind of trait. The amount and type of melanin produced in the iris is controlled by several genes (such as OCA2 and HERC2) inherited from both parents. This genetic makeup is set at conception — pigmentation can go on developing through early infancy as melanin deposits, but which colour a person ends up with is governed by which genes they inherited, not by where they live, how they are raised, their hormone levels, or the society around them.

• Environment covers surroundings, climate, and upbringing, which shape acquired habits and skills over time, an entirely different category of trait from genetically fixed pigmentation.

• Hormones regulate growth, metabolism, and mood, and their levels rise and fall through life, but they play no part in which pigment genes a person inherited or how much melanin the iris carries.

• Society shapes a person's behaviour, values, and conduct through learning and interaction, an influence on conduct rather than on a trait fixed by inheritance before birth.

Since eye colour traces back to genes inherited from one's parents rather than to surroundings, hormones, or social life, heredity is the factor being asked for — a classic example used in pedagogy and psychology to illustrate the biological basis of individual differences among students.