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Why are many human traits influenced by multiple genes?

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Diagram of a DNA double helix showing its twisted ladder structure, with a close-up of the sugar-phosphate backbone, paired nitrogenous bases (adenine, thymine, cytosine, guanine), and labeled nucleotides and hydrogen bonds. — human genome The human genome is made up of approximately three billion base pairs of deoxyribonucleic acid (DNA). The bases of DNA are adenine (A), thymine (T), guanine (G), and cytosine (C).

Many human traits are influenced by multiple genes through a process known as polygenic inheritance. Unlike traits determined by a single gene, polygenic traits are governed by several or many genes, each of which contributes a relatively small, cumulative effect to the final phenotype.12

This mode of inheritance is responsible for quantitative or continuously varying characteristics—such as stature, blood pressure, and intelligence—which range across a spectrum rather than appearing in distinct, categorical forms. Because these traits are determined by multiple interacting genes, they are also frequently influenced by environmental conditions, making the final expression of the trait a result of the complex interaction between nature and nurture.134

Additionally, susceptibility to certain congenital malformations follows this polygenic pattern. In these cases, while the defect itself may appear as a discrete condition, the underlying risk is continuously variable, manifesting only when a specific developmental threshold is exceeded due to the combination of polygenic susceptibility and environmental factors.1