What percentage of people become great-grandparents?

To what extent is service life a matter of predisposition?

How long we live on average depends on our way of life and the environment, but also on genes: Longevity often runs in the family - at least that is a widespread assumption. But what role do genetics actually play in lifespan? A study based on the family tree of more than 400 million people came to the conclusion: The genes probably make up no more than seven percent.

Why are some people 120 years old and others not even 70? And what makes some people stay fit into old age, while others have to deal with ailments before they reach retirement age? Scientists have long been looking for the biological basis for our lifespan. It is clear that the way of life and the environment play an enormous role in this. We live significantly longer than our grandparents and great-grandparents because nutrition, medical care and other factors are better today. At the same time, however, heredity seems to be important for our age: "It is widely assumed that a long life runs in the family," explain Graham Ruby of Calico Life Sciences in San Francisco and his colleagues. If the grandparents and parents grew very old, this awakens the hope in many people that they themselves will be among the long-lived.

Data from 400 million people

But how much of our lifespan is genetically determined? To create more clarity, Ruby and his team have now evaluated one of the largest data sets on this topic to date. They used the fact that there are now more and more genealogy portals on the Internet through which people can find out their ancestry. One of these portals, Ancestry, contains the family data of 54 million members and their roughly six billion ancestors.

In collaboration with the Ancestry operators, the researchers selected the data of around 400 million people and their relatives who lived in the 19th and 20th centuries. With the help of mathematical and statistical processes, they evaluated this data for the longevity of the people, but also for their degree of relationship and possible socio-cultural similarities. Because if siblings live and grow up in one household, this too can equalize their lifespan. “Sociocultural factors that affect lifespan can be passed on and shared within families, as can genetic factors,” explain Ruby and his team.

Disruptive factor partner choice

And indeed: The evaluation showed that the heritable life expectancy is apparently even less than previously assumed. The scientists came for the influence of the genes to a value of a maximum of seven percent. Even this value could be shaped by socio-cultural influences, so that the pure effect of the genes may be even lower, as Ruby and his colleagues explain. But why were these values ​​so much lower than previous estimates? One explanation is an effect that biologists call "assortive pairing". It comes about because we humans tend to choose partners who are similar to us in many ways. In their family tree analyzes, the researchers noticed that not only spouses often lived for a similar length of time, but also in-laws, brothers-in-law and other relatives by marriage who did not live in the same household.

In the opinion of the scientists, this suggests that we apparently unconsciously prefer partners - and their families - who have a similar lifespan to ourselves due to lifestyle, biology or other factors lead to a similar lifespan for the spouses. Or the origin from the same area or a certain body stature. This assortive pairing then indirectly ensures that the lifespan aligns within families. Because previous studies have not taken this factor into account, they set the purely genetic inheritance of lifespan correspondingly higher, according to the explanation by Ruby and his colleagues. If you take this strongly sociocultural component out of the equation, however, you arrive at a significantly lower influence of genes on longevity.

This means: Even those who come from a family of rather long-lived people cannot rely on inheritance as a life extender. The positive thing, however, is that our lifespan is not that predetermined.

Source: Graham Ruby (Calico Life Sciences LLC, South San Francisco) et al., Genetics, doi: 10.1534 / genetics.118.301613

November 24, 2020

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