Factors Influencing Development

People clearly differ from one another — in skin, hair and eye colour, in height, in temperament, and in psychological traits like intelligence, learning ability, memory and personality — yet everyone still belongs to the same species. The answer to why people are both so different and so alike lies in the interaction between heredity and environment. Every cell in the body carries a genetic code inherited from one's parents; this human genetic code is why a fertilised human egg always develops into a human being and never into any other species. Genetic transmission is complex: most observable human characteristics arise from combinations of a very large number of genes (commonly cited as 80,000 or more), so an enormous variety of characteristics and behaviours is possible, and no individual carries or expresses every characteristic their genes could theoretically produce. A person's actual genetic material — their full genetic inheritance — is called the genotype. Not all of the genotype is visibly expressed, however; the phenotype is the way a person's genotype actually shows up in observable, measurable characteristics — physical traits like height, weight, eye and skin colour, as well as psychological traits like intelligence, creativity and personality. The phenotype we actually see in a person is the joint result of their inherited genotype interacting with their environment.

Genes provide a distinctive blueprint and timetable for development, but genes never operate in isolation — development always takes place within the context of a person's environment, and this interaction is what makes every individual unique. Consider a child whose genotype predisposes them toward introversion, growing up in an environment that actively promotes social interaction and extroversion — such an environment can make the child somewhat more extroverted than their genetic starting point alone would predict. On the other hand, environment cannot override every genetic limit: a person with genes for 'short' height will never grow taller than their genetic ceiling allows, no matter how good their nutrition — genes set the outer limit, and within that limit the environment shapes the outcome. …