During fertilization, half of the father's genome is mixed with half of the mother's genome in order to form the diploid genome of the new individual. The result is the individual's genetic make-up or genotype. The genotype is completely unique to each individual, a one-of-a-kind combination of parental genetic material. The only exception to this rule is identical twins that originate from a single fertilized egg. Their genetic material is completely identical.

Studies have shown that identical twins can develop differently if they grow in different environments. For example, nutrition, its quality and quantity, as well as the stimuli from the growing environment, can form different kinds of individuals from the same genotype. Although individuals have the same genotype, they may have unique ways of expressing that genotype. The individual expression of a genotype is called a phenotype.

A phenotype consists of an individual's observable traits. Some traits are determined by the genotype, while other traits are determined by environmental factors. Heredity defines the boundaries within which an individual’s characteristics can change, and the environment modifies the characteristics within those boundaries.

Environmental factors include not only nutrition, but also, for example, the chemicals to which an individual is exposed to during their development and growth, as well as their personal experiences. These factors affect the development of an individual right from fertilization.