Genetics plays a less important role in life than we thought

Romantic scenes that never happen: your eyes meet. Your heart is pounding. This is the person you are sure of, because you are convinced that she will live to 95 years. That's what you've always dreamed of.

Lifespan is not usually on the list of people looking for what they are looking for in a partner. But, according to an article published this week in the newspaper GeneticLongevity is closely linked to marital relationships, which means that people are good enough to choose partners who live a similar life. By ignoring this behavior, estimates of the genetic contribution to longevity have been significantly overstated.

I knew the second time I saw your blood

Nobody chooses partners according to their lifetime. As the authors of the document wisely note, life expectancy "can not be observed until death, when the opportunity to marry is over." But as all those who have already gone out can tell you, people are likely to marry their partner) in such characteristics as wealth and education, which play an obvious role in longevity.

J. Graham Ruby, the lead author of the paper, works for Calico Life Sciences, a research and development company founded by Alphabet. Calico's mission is to harness advanced technologies to better understand the biology that controls lifespan. Ruby has used huge amounts of data from Ancestry.com to study the role of genes in the lives of more than 400,000 people born in the 1800s and beyond. early 20th century.

For complex traits such as lifespan, a large number of genes will play a role, as will multiple environmental factors. The role of genes is therefore described according to the variability that can explain. Estimates of genetic influence are around 15 to 30%, which means that up to 30% of the observed variation in the length of human life can be explained by genetic differences between individuals.

The estimates vary in part because of differences in data sources and methods of calculation and partly because the statistics will not be the same from one population to another: countries differ in the most common causes of death, the environmental risk factors people face, and population differences. are exposed to the same risk factors. For example, in a poor country where the risk of contracting an infectious disease and dying in childbirth is high, the few wealthy citizens can avoid these risks through expensive health care. This will be very different from a rich and egalitarian country where cancer is one of the leading causes of death.

Your life is correlated with the brother-in-law

The calculations are complicated, but the thinking behind them is simple: when genes play a role in a trait, you should be quite similar to your siblings and your parents, a little less like your cousins, less similar to your secondary cousins, etc. sure. Ruby and his colleagues used the family tree data to determine if life spans were similar between close and close relatives. This produced estimates of heritability similar to those previously calculated: the siblings' life span was strongly correlated, the first cousins ​​a little less correlated, and so on.

But the life of the spouses was also correlated. This could be easily explained by spouses sharing the same household and lifestyle: eating the same healthy diet or eating cigarettes together. But researchers have noticed something odd: the lifespan of other relatives related only by marriage also related to. This can not be explained by genes or a shared environment.

Thus, Ruby and his colleagues began to investigate the life of the in-laws. They then studied first-time siblings, then more distant relationships, such as "the brother or sister of a brother's spouse" (the sister of your brother's wife) and "the spouse of a brother". A brother or sister of a spouse "(the husband of your husband's sister). Even in these distant relationships, the life span was correlated: if your brother's or sister's spouse lived to old age, this means that you are a little more likely to do the same.

What happens here is an assortative mating: it is likely that people marry people who resemble them for certain characteristics, such as education and wealth, and these characteristics are in turn related to longevity. This high level of assortative mating has greatly inflated the estimate of heritability. Once this has been taken into account, Ruby and his colleagues have achieved a much lower genetic influence figure of 7%, at most. These seven percent include both genetics and inherited non-genetic traits, such as healthy or unhealthy habits that parents pass on to their children. These two things can be hard to separate, but it's fair to assume that the only contribution of genetics is even weaker.

This is an estimate that can change, and probably already has, because this study was based on a set of historical data of people born in very different health landscapes and very different marriage practices. The Ancestry.com database also contains a very large amount of data on American families of European origin, which limits the generalization to other groups, countries, cultures and eras. But this is an essential observation for thinking about heritability statistics in general, because assortative mating covers all kinds of characteristics. There is also a negative assortative coupling, also known as "opposites attract", to take into account.

This may seem disappointing for a company interested in "life-cycle biology". But of course, the processes of aging and disease remain biological, even though the causes of these processes lie less in our genes than previously thought. In fact, looking for what constitutes the vast majority of differences in the life span of a person can be precisely what gives the most interesting answers.

Genetic, 2018. DOI: 10.1534 / genetics.118.301613 (About DOIs).