I recently received an email from a company called MyGeneProfile: "By discovering your child's inborn talents & personality traits, it can surely provide a great head start to groom your child in the right way ... our Inborn Talent Genetic Test has 99.8% accuracy." I'd registered to receive information from the company having heard it was offering a genetic test for such diverse traits as optimism, composure, intelligence and dancing.

Despite all the efforts of the Human Genome Project, I was not aware of any genetic test that could reliably predict a child's personality or ability. I was not therefore surprised when my emails asking for evidence went unanswered, though I continue to receive messages that oscillate between carrots (free gifts! discounts!!) and sticks (without this test "your child will have MISERABLE life [sic])".

The company relies on a widespread assumption that people's mental and physical attributes are predictable from their genes. So where does this belief come from, and is it wrong?

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.

This means that we get a very different impression of the strength of genetic influences on a trait if we look at the impact of a person's whole genome, compared with looking at individual genes in isolation.

The twin study was the traditional method for estimating genetic influences before we had the technology to study genes directly, and it compares how far people's similarity on a trait depends on their genetic relationship. Researchers measure a trait, such as sensation-seeking, in identical and fraternal twin pairs growing up in the same environment, and consider whether the two twin types are equally similar.

If both sets of twins resemble each other equally strongly, this indicates that the environment, rather than genes, is critical. And if twins don't resemble one another at all, this could mean either that the trait is influenced by experiences not shared by the co-twin, or that our measure of sensation-seeking is unreliable.

If identical twins are more similar than fraternal twins, this means genes affect the trait, ie it is heritable. There are several niggly criticisms of the twin method; for instance, it can give misleading estimates if identical twins are treated more similarly than fraternal twins (in other words they have more closely matched environmental influences), or if twinning itself influences the trait in question. For most traits, however, these don't seem sufficient to explain away the substantial heritability estimates that are found for traits such as height, reading ability and sensation-seeking.

But these estimates don't tell us about the individual genes that influence a trait – they rather indicate how important genes are relative to non-genetic influences.

Interactive effects, either between multiple genes or between genes and environment, will not be detected in a twin study. If a gene is expressed only in a particular environment, twins who have the same version of the gene will usually also have the same environment, and so the expression of the gene will be the same for both. And for an effect that depends on having a particular combination of genes, identical twins will have the same constellation of genetic variants, whereas the likelihood of fraternal twins having an identical gene profile decreases with the number of genes involved. Heritability estimates depend on comparing the similarity of a trait for identical versus fraternal twins, and will be increased if gene-gene interactions are involved.

In contrast, genome-wide association studies are designed to find individual genes that influence specific traits. They look for associations between DNA variants (alleles) and the trait, either by categorising people, eg as dyslexic or not, and comparing the proportions with different alleles, or by seeing whether people who have zero, one or two copies of an allele differ in their average score on the trait. When these studies started out, many people assumed we would find gene variants that exerted a big effect, and so might reasonably be termed "the gene for" dyslexia, optimism, and so on. However, this has not been the case.

This account may surprise readers who have read of recent discoveries of genes for conditions such as dyslexia. The reason is that when very large samples are used, it is possible to detect even weak effects. In reports of molecular genetic studies, the statistic that is most often emphasised is the p-value, ie how probable it is that a result could have arisen by chance. A low p-value indicates that a result is reliable, but it does not mean the effect is large.

Consider one of the more reliable associations between genes and behaviour: a gene known as KIAA0319 which has been found to relate to reading ability in several different samples. In one study, an overall association was reported with a p value of 0.0001, indicating that the likelihood of the association being a fluke is 1 in 10,000. However, this reflected the fact that one gene variant was found in 39% of normal readers and only 25% of dyslexics, with a different variant being seen in 30% of controls and 35% of dyslexics.

Some commentators have argued that such small effects are uninteresting. I disagree: findings like this can pave the way for studies into the neurobiological effects of the gene on brain development, and for studies of gene-gene and gene-environment interactions. But it does mean that talk of a "gene for dyslexia", or genetic screening for personality or ability, is seriously misguided.

What are the implications of all this for the stories we hear in the media about new genetic discoveries? The main message is that we need to be aware of the small effect of most individual genes on human traits. The idea that we can test for a single gene that causes musical talent, optimism or intelligence is just plain wrong. Even where reliable associations are found, they don't correspond to the kind of major influences that we learned about in school biology. And we need to realise that twin studies, which consider the total effect of a person's genetic makeup on a trait, often give very different results from molecular studies of individual genes.

Dorothy Bishop is a professor in developmental neuropsychology at the University of Oxford and blogs at BishopBlog

Background reading

Bishop, DVM (2009) Genes, cognition and communication: insights from neurodevelopmental disorders. The Year in Cognitive Neuroscience: Annals of the New York Academy of Sciences, 1156, 1-18

Maher, B (2008). Personal genomes: The case of the missing heritability. Nature, 456, 18-21

Plomin, R, DeFries, JC, McClearn, GE and McGuffin, P (2008). Behavioral Genetics. (5th Edition). New York: Worth Publishers

Rutter, M (2006). Genes and Behavior: Nature-Nurture Interplay Explained. Oxford: Blackwell