杏吧原创

SOMEWHERE out there is a wonderful woman who has donated 10 of her eggs to help me create a family. If I decide to use them, I could give birth to a child with whom I do not share a genetic history. This inevitably makes me wonder: how much does that genetic bond matter?

Special: Modern families

Today鈥檚 families are more diverse than ever. We explore what this means for our relationships and our children

We鈥檝e all heard stories of twins who were separated at birth and reunited as adults, only to find their personalities bear uncanny resemblances. Over 20 years of looking at such stories, the Minnesota Study of Twins Reared Apart came to a remarkable conclusion: the personalities of identical twins raised separately are just as similar as if they had been raised together. A suite of genetic studies supports this finding, showing that our DNA helps shape all aspects of our identities from intelligence to risk-taking and even our political beliefs.

鈥淭he genetic influence on individual differences in psychological traits is so widespread that we are unable to name an exception,鈥� says Robert Plomin, a behavioural geneticist at King鈥檚 College London.

That isn鈥檛 to say that biology is destiny, says Laura Baker, who studies human behaviour at the University of Southern California. We know, for example, that warm and supportive parents are more likely to raise better-adjusted adolescents, and that children who experience early trauma are more likely to develop depression or anxiety, or show antisocial behaviour.

Studies consistently attribute around half of the differences in our personalities to genetic effects. Plomin, for instance, recently found that genes account for 52 per cent of the differences between children鈥檚 exam scores.

But it鈥檚 not that simple. How children react to environmental pressures like stress is determined in part by their genes, something many studies don鈥檛 control for. Take, for example, a 2013 study on communication. It found that children who are spoken to directly tend to develop better language skills, but didn鈥檛 take into account the fact that their genes could influence how they respond to speech. In other words, if parents talked exactly the same way to an adopted sibling, they might see a different outcome 鈥� which they wouldn鈥檛 if the effect was purely environmental.

One study that did control for the effect of genes looked at the correlation between maternal negativity and antisocial behaviour in adolescents. It found that two-thirds of the correlation can be attributed to genes.

Egg and sperm race

On the other hand, it turns out that there鈥檚 more to genetics than the egg and sperm. A woman can influence her child鈥檚 genes even when her DNA doesn鈥檛 play the starring role. We now know that when an embryo burrows into the uterus, it encounters tiny molecules from its birth mother 鈥� called microRNA. These switch genes on and off and could influence how the fetal brain grows and they therefore have the potential to affect a child鈥檚 development, although we still don鈥檛 know exactly how. The effect of microRNA is relevant to parents considering surrogacy as well as donor recipients.

As a recipient of a donor egg, I may even pass on my DNA to my child. During pregnancy, mothers and their embryos exchange cells that take up residence in many different organs. We know that in mice, fetal cells form neurons in a mother鈥檚 brain. And we know that a mother鈥檚 cells can influence her child鈥檚 insulin production and immune function. We each probably only inherit a small number of these cells, but since they have the potential to stick around for a while, my cells could influence my child鈥檚 health for decades.

鈥淚 would think that it provides some comfort to you, to know that you are providing some genetic material to your donor-conceived child,鈥� says Lee Nelson at Fred Hutchinson Cancer Research Centre in Seattle, Washington. It came as a surprise to me, but it does indeed provide comfort. I have never felt any great urge to reproduce for the sake of replicating my own genes, yet given their importance, it鈥檚 nice to know I might play some direct role in this area. I may even pass on protective cells I鈥檝e inherited from my own mother.

鈥淚鈥檓 of the belief that a small number of maternal cells we pass over during pregnancy have a lot of important effects throughout our lives,鈥� says Nelson.

In the end, how much does any of this really matter to forming healthy family relationships? Numerous studies show that relationships in donor-conceived families are just as likely to be positive as in families who are genetically related (see 鈥�How changing families are affecting our children鈥檚 well-being鈥�). Fiona MacCallum at the University of Warwick, UK, has shown that maternal warmth and sensitivity are the same whether children are genetically related, donor conceived or adopted. And mothers of non-genetically-related children are no more likely to report difficulties 鈥� although growing evidence suggests that being open about a child鈥檚 origins as early as possible is key to forming healthy relationships (see 鈥�Do you have a right to know who you are?鈥�).

To any future children of my own, I can give only one piece of information about their anonymous relative. And that is that she has performed a great act of kindness 鈥� and if that鈥檚 a trait they are likely to inherit, it鈥檚 not a bad place to start.

This article appeared in print under the headline 鈥淗ow much do my genes matter?鈥�