In this chapter we’ve focused almost entirely on genetic patterns that result in monoallelic traits—traits in which a change in a single gene is enough to cause a change in phenotype. But monoallelic inheritance patterns are rare.
Most phenotypes are controlled by multiple genes. In general, when people talk about inheritance, they are likely discussing complex traits such as appearance, intelligence and personality. Commonly we hear phrases like “She’s got her father’s eyes” or “He’s got his mother’s temper.” Neither eye color nor temper are controlled by single genes. In fact, most of the meaningful traits that would influence an organism’s ability to survive and thrive are controlled by many separate genes. A complex trait such as intelligence, for example, requires the coordination of many biological functions and thus many genes working together in concert with environmental influences. Some have suggested that there are over 500 separate genes that may control human intelligence. . Traits controlled by many genes are called polygenic traits.
Because polygenic traits represent a mixture of genes, it is very difficult to find single genes that can significantly change a polygenic trait. Psychologists have developed detailed ways of measuring complex traits such as intelligence and mental illnesses. However, it’s much more difficult to study how these traits are passed down genetically. One such example is the disorder schizophrenia, an illness characterized by peculiar social behavior and confusion about reality. Schizophrenia is believed to be caused by a mixture of inheritable genetic mutations and triggers from the environment. People are more likely to have schizophrenia if they have a relative who has it, which suggests genetics play a role in susceptibility. But schizophrenia is also a polygenic trait and difficult to track genetically.