Dr. Karen Mohlke is associate professor of genetics and a member of the Carolina Center for Genome Sciences at the UNC School of Medicine. She explains how the genetics of common traits such as height and eye color is much more complicated than the pea-plant genetics we might remember from our old textbooks. Questions and answers have been edited.
Q: What is known about the genetics of height?
You have probably heard of genes that act in a recessive or dominant manner, meaning that either one or two copies of the "taller" version of a gene contribute the same amount to height. But many genes may have an additive effect, meaning that two copies of the "taller" version increase height more than one copy. And many genes are involved, which is why you can't just look at the height of mom and the height of dad and perfectly predict the height of their offspring. So one gene may increase your height a tiny bit, another may increase it even more, and so on.
Q: How many genes appear to play a role in height?
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We recently pooled our results together with 45 other research groups, looking across the entire genome for patterns that link genes and height in 183,723 individuals. We just published our results in the journal Nature, showing that there are at least 180 genes that influence human height. To my knowledge, this is also the largest number of gene locations that have been identified containing common variants that are associated with any trait or disease.
Q: So do we now know all of the genetics of height?
Height is very highly heritable, meaning that it runs in families. From population studies, we know that about 80 percent of human height is due to genetics. We estimate that the genome regions we have identified in this recent study account for about 10 to 15 percent of variation in human height. So what's left - the difference between that 80 percent and the 15 percent - is what is called the "missing heritability." It may be that rare gene variants, which are even more difficult to identify than the ones we found here, are also contributing to the heritable variation.
Q: Why study the genetics of height? Height is a classic complex trait that gives us insight into the genetics of other complex traits. I know that there are groups studying hair color and eye color, but the large consortia that we are involved in are more focused on diseases and traits related to those diseases. Other traits that we study are cholesterol levels and glucose levels and insulin levels, Type 2 diabetes and blood pressure. I have chosen to study complex traits because I find them more intriguing, with their combination of multiple genes and the role of the environment. It is definitely the cutting edge of genetics research.