7.11: Multigene traits: how are continuously varying traits such as height influenced by genes?

When babies are born, the parents are often curious about how tall their child will grow to be. Old wives’ tales suggest a couple of ways for predicting height. If the baby is a boy, they say to add 5 inches to the mother’s height and average that with the father’s height. If it is a girl, subtract 5 inches from the father’s height and average that with the mother’s height. Alternatively, the lore says, just take the child’s height at two years of age and double it.

These methods can be surprisingly accurate at making predictions, but they don’t really help us understand the underlying reason why height can be predicted so well. The reason these methods work is that genes play a strong role in influencing height, and so offspring do resemble their parents in measurable ways. But unlike the simple case of Mendel’s pea plants, where a single gene with two alleles determines the height of the plant, for humans and most animals, adult height—a continuously varying trait—is influenced by many different genes. Such a trait is said to be polygenic.

Recent research has identified at least 180 heritable loci that influence adult height. For each of these loci, the alleles that a person carries play a role in determining his or her height. Individuals with “tall” variants for more of the genes tend to be taller than those with the “tall” alleles for fewer of the genes. The term additive effects describes what happens when the effects of the alleles of multiple genes all contribute to the ultimate phenotype.

Of the genes influencing height, a large number play roles in skeletal growth and hormone pathways. One of these height genes, for example, is on chromosome 15 and codes for an enzyme that converts testosterone to estrogen. This enzyme influences height because estrogen helps bones fuse at their ends and thus stop growing. The variety of heights seen among humans—from very short to very tall, with every height in between—reflects the fact that height is a trait influenced by contributions from multiple genes, as well as the environment (FIGURE 7-23).

Many other physical traits are influenced by multiple genes, including eye color in humans. Eye color was long believed to be controlled by a single gene with a dominant brown-eye allele and a recessive blue-eye allele, but it now seems that eye color is the result of the interactions between at least two genes and possibly more—a situation that makes more sense, given the significant continuous variation in eye color seen among adults.

Many behavioral traits are influenced by multiple genes. The developmental disorder known as autism, for example, seems to be the result of alterations in numerous—perhaps as many as 10 or even 20—different genes. Individuals with autism have difficulty interacting with others, particularly in making emotional connections. Autistic individuals also tend to have narrowly focused and repetitive interests. Autism is notoriously difficult to study, though, because its symptoms are varied, as is their intensity. It turns out that these variations in the disorder may result from the different combinations of the many genes involved in autism, much as different alleles for the many height genes may work together to produce a variety of heights.

Interestingly, the genes responsible for autism may also influence some desirable characteristics. This idea—called the “geek theory of autism”—was first suspected when an upsurge in cases of autism in children was noticed in areas with large numbers of high-tech workers, including Silicon Valley in California and near Cambridge, England, in the heart of the United Kingdom’s high-technology industry. Subsequently, researchers have published findings showing an over-representation of autistic children among parents working in the fields of engineering, physics, computer science, and math (although the researchers point out that autistic children are born to parents across all professions and socio-economic backgrounds). Perhaps alleles for genes that contribute to making individuals good at computer programming and solving complex technology problems can produce autism when someone carries too many of them.