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he psychic and physical characteristics of human beings, and the differences between individuals, are the consequence of an interaction between the genes that are present in the fertilized egg and the sequence of environmental circumstances that the developing organism experiences during its entire life history. With a few exceptions, like cystic fibrosis where possession of the defective genotype leads ineluctably to the disease, or language acquisition where the language spoken depends only on experience and not at all on genotype, human characteristics are all subject to this interaction of forces. There are, moreover, random events in cell growth and differentiation that are neither genetic nor environmental in the usual sense, and which play an extremely important part in development, especially in behavioral traits. (en) |
