Researchers at Stanford University recently reported that they discovered a genetic basis for blondes. At first glance, this result could be a poster child for how we typically think genetics works: Brown-haired Europeans have an "A" at one spot in their DNA, while many blonds have a "C" instead. Blonds are common in Northern Europe, where many people carry the "C" DNA variant, but this variant is almost absent in the rest of the world. The researchers found that the "C" variant occurs in a crucial spot in the genome where a control protein attaches itself to DNA and activates a gene involved in pigmentation. In mice with the blond "C" mutation, the gene's activity is down in hair follicles and the mice have lighter fur. The study offers a neat molecular explanation for one of the most visually striking examples of genetics.
This may sound like a case of textbook genetics, similar to the yellow peas and pink Snapdragons that you may vaguely remember from high school. But these visually obvious examples are atypical, and they give the wrong impression of how we are shaped by our genes. Hair color, while not exactly the simplest case (it is affected by several genes, not just the one studied by the Stanford team), is strongly determined by genetics. You can dye your hair to mask its color, and it may go gray as you age, but your natural hair color depends on your DNA, and not on your culture, your climate, how much you exercise, or what your mother ate when she was pregnant with you. That is obviously not true of most of our other traits, especially behavioral ones. Most of our traits are influenced by both our DNA and our environment, and relatively simple ones like hair color are not a good guide to how we should think about the effects of our genes.
The often indistinguishable effects of genes and environment are important to keep in mind especially when we read about genetic studies of behavioral or mental traits, which, unlike many physical traits, can be directly transmitted from parents to offspring, independently of genes.
THE TANGLED ROLES OF genes and environment were recognized right from the start of the modern science of genetics. One of the clearest statements of the problem was made in 1911 by Wilhelm Johannsen, the scientist who coined the word "gene." Johannsen laid out the paradox at the heart of genetics that makes it so difficult to pin down the role that genes play in in our lives. The paradox is this: Organisms that have the same genes can still have different traits. And organisms that share the same traits may in fact have different genes.
To explain why this is so, Johannsen built on a central insight that went against thousands of years of thinking about heredity, from Hippocrates to Darwin. Despite the fact that offspring resemble their parents, parents don't directly transmit their traits to their children; they transmit their genes. "The personal qualities of any individual organism do not cause the qualities of its offspring," Johannsen wrote, "but the qualities of both ancestor and descendant are in quite the same manner determined by the nature of the 'sexual substances.'" Those sexual substances are little more than vehicles for passing on genes.
At the time, nobody knew what a gene was made of, but Johannsen argued that, whatever their ultimate constitution, genes work by influencing the chemistry of our cells—and that chemistry is malleable. The chemical action of a gene is in one sense hard-coded by its molecular make-up, but, like any other chemical, its action is also influenced by the environment. Just as the ability of salt to dissolve depends on the temperature of the water, the chemical consequences of a gene depend on the environment, Johannsen argued, as well as on the chemistry of the other genes present in the organism.
So while you inherit your genes from your parents, those genes will not necessarily manifest themselves in the same way in your body. And most crucially, as Johannsen showed in his experiments on bean plants, differences in traits that are caused by slight variations in the environment can be indistinguishable from differences that are caused by genetics. As he wrote, "by simple inspection of a series of different individuals it will be quite impossible to decide if they have or have not the same genotypical constitution."
The often indistinguishable effects of genes and environment are important to keep in mind especially when we read about genetic studies of behavioral or mental traits, which, unlike many physical traits, can be directly transmitted from parents to offspring, independently of genes. A trait that is mostly influenced by genetics in one population may be more strongly influenced by the environment in a different population. And a changing environment can account for rapid changes in highly heritable traits, most famously in the case of the Flynn effect and IQ. There is no question that our genes play a role in our behavior, but to tease apart genes from environment requires careful experiments coupled with the formidable mathematical toolbox of modern genetics. And even then, it's often very hard to do.
The Stanford team's work on the genetics of blondes was a rare, impressive example of nailing down the DNA variant behind a particular trait. This was a difficult achievement for a straightforward feature like hair color. It shows just how daunting it is to understand the role that genes play in our lives, especially in something as complex and culturally-shaped as human behavior.