After dropping a pair of male and female adult rats into a rectangular Plexiglas container, Frances Champagne can expect one of a few scenarios to ensue. The male will definitely try to mate with the female — but the female is less predictable. She might approach him, appraise his scents and arch her back to allow him to mount her. Should a second male enter the cage after she's mated with the first, she may be similarly hospitable.Genetics by itself is infinitely more complex than we would think. Here is an interesting article by Dorothy Bishop on how misleading the idea of one-gene-one-trait is.
Some females play it coy, however, evading the male, requiring more courtship and, if mating does occur, avoiding another go. A number of factors can influence what the female does, but to Champagne, a behavioural scientist at Columbia University in New York, one is particularly beguiling: how often the female rat's mother licked and groomed her during her first week of life. Doting mothers have prudish daughters, whereas the daughters of inattentive rats cavort around like mini Mae Wests. At the heart of these differences lies the sex hormone oestrogen, which drives female sexual behaviour. Champagne says that neglected rats might respond to it more strongly than those raised by attentive mums.
The phenomenon is just one example of how experiences early in life can shape behaviour, and it may apply to humans. It is known, for example, that children who grow up in poverty are at greater risk as adults for problems such as drug addiction and depression than those with more comfortable upbringings, regardless of their socioeconomic situation later in life. But what is it about early experiences that has such a lasting effect? For Champagne and many of her colleagues, the answer has been apparent for nearly a decade. Life experiences alter DNA; not necessarily its sequence but rather its form and structure, including the chemicals that decorate it and how tightly it winds and packs around proteins inside the cell. These changes, often referred to as epigenetic modifications, make genes easier or more difficult for the cell's protein-making machinery to read.
People's understanding of genetic effects is heavily influenced by the way genetics is taught in schools. Mendel and his wrinkly and smooth peas make a nice introduction to genetic transmission, but the downside is that we go away with the idea that genes have an all-or-nothing effect on a binary trait. Some characteristics are inherited this way (more or less), and they tend to be the ones that textbooks focus on: for example eye colour, colour-blindness, Huntington's disease. But most genetic effects are far more subtle and complex than this. Take height, for instance. Genes are important in determining how tall you are, but this is not down to one gene: instead, there is a host of genes, each of which nudges height up or down by a small amount.
Furthermore, genetic influences may interact in complicated ways. For instance, coat colour in mice is affected by combinations of genes, so that one cannot predict whether a mouse is black, white or agouti (mouse coloured!) just by knowing the status of one gene. The expression of a gene may also depend crucially on the environment; for instance, obesity relates both to calorie intake and genetic predisposition, but the effects are not just additive: some people can eat a great deal without gaining weight, whereas in others, body mass depends substantially on food intake. And a genetic predisposition to obesity can be counteracted by exercise.