Children who don't inherit a particular form of a gene can still be shaped by its presence in their family, a paper in Science demonstrates. The authors compared the genomes of 21,000 Icelanders with their parents and measurable outcomes. They introduce the term “genetic nurture” to describe a phenomenon that breaks down the walls between the old concepts of nature and nurture.

We receive one version (or allele) of each of our genes from each parents (male sex chromosomes aside). These genes represent our fundamental architecture, on which our environment acts to shape our attributes and behavior. Epigenetics, which shows how the environment can change the extent to which a gene is expressed, complicates this neat division of nature versus nurture, but Dr Kári Stefánsson of deCODE Genetics suggests things are even more complex still.

Attempts to distinguish the contributions of nature and nurture look at inherited alleles. The versions of genes that a parent, let alone a grandparent, had that were not transmitted have been treated as irrelevant.

Yet these alleles will shape parental behavior, and therefore the conditions in which a child is raised, indirectly influencing the child's outcomes. Stefánsson told IFLScience that to ignore this effect means estimates of the heritability of certain traits through projects such as comparisons on identical and non-identical twins will be exaggerated. He is confident this has occurred in such contentious areas as the heritability of IQ.

To understand genetic nurture, imagine an allele associated with strong reading skills. A parent with this allele is likely to fill their home with books and encourage their children to share their passion for reading. Even if a child does not inherit the allele, provided they are raised by this parent, they will probably do more reading than others their age and become a better reader. The paper speculates the effect will be reinforced if siblings inherit the allele, influencing those that do not, but this has yet to be investigated.

Although genetic nurturing makes intuitive sense, at least for some traits, it has largely been ignored until now, and certainly not quantified. Stefánsson and fellow authors write: “In animal studies, it is well established that alleles in a parent that are not transmitted to the offspring can nonetheless influence the offspring’s phenotypes. Most examples involve effects manifested at the fetal stage, at which only the non-transmitted maternal alleles are relevant.”

Stefánsson and co-authors confirm the effect using Iceland's enormous database of gene sequences and characteristics, controlling for cases where children were adopted or otherwise not raised by their biological parents. They found non-transmitted alleles known to be associated with educational attainment accounted for an impressive 30 percent of the effect of genes that are transmitted.

The same database revealed the influence of genetic nurture in smoking rates, BMI, and cholesterol levels. Stefánsson told IFLScience that while the way these influences were passed on has not been studied, “it has to be through the brain”. Thus an allele that gives a parent a taste for particular foods or a disinclination to exercise will alter their behavior and therefore the environment in which their children grow up.

In some cases, the authors found equal effects from the mother and father, whereas for others, the presence of the allele in the mother was more influential. The effect was evident even when controlling for assortative mating, the tendency for people to have children with those similar to themselves.

It is acknowledged that the age at which people become parents can affect the chance their children will have certain conditions or traits, and non-transmitted alleles from the mother have been considered in this light. However, Stefánsson argues this is only the tip of the iceberg and that other forms of genetic nurture have been ignored up until now.

A commentary paper in the same edition of Science expresses concern that genetic nurture invalidates some existing techniques used by geneticists to identify genes that influence particular traits and behaviors. However, Stefánsson told IFLScience he does not believe such methods will need to be abandoned, but that those practicing them will have one more thing they need to account for if their results are to be reliable.