Source: WeChat official account "Chunyu Health Science Popularization (chunyuedu)"
Although marriage with cousins has become a taboo in modern society, it is not uncommon in history.
Eleanor, the wife of Franklin Delano Roosevelt, the 32nd President of the United States, is his cousin. The wife of a great scientist like Darwin is his cousin, who is one year older than him.
Throughout human history, for a long time, society did not think it was immoral or immoral to do so. For a long time, marrying cousins was considered the best choice.
Yaniv Erlich, a data scientist at Columbia University, published research data on gene pedigree of 130 million family members in Science magazine. Through analysis, researchers can find out the specific time range when society does not advocate marriage with cousins and the genetic correlation between married couples.
The ancient people married close relatives because of the traffic inconvenience?
According to the experimental data provided by Erlich, between 1650 and 1850, a quarter of people finally married their cousins.
Many people even marry their first siblings directly. However, this situation has changed significantly within a century. According to Erlich, by 1950, the average correlation between married couples was the seventh representative.
A common sense explanation for the change is the improvement of traffic. Convenient transportation makes it easier for single men and women to access more potential marriage partners. There is a theoretical basis for this, because before 1950, most people had a very limited range of activities, and most people would eventually choose to marry people within a radius of six miles (about 9.6 kilometers) of their birthplace, so most people married cousins who are close relatives.
But other factors may also indirectly cause this phenomenon.
Erlich said that large-scale genetic data analysis shows that many people continue to marry their cousins even after the industrial revolution has significantly improved traffic mobility. One reason is that consolidating money power and family identity is an important part of family marriage.
Erlich believes that the transformation of social form and the emergence of marriage taboos for cousins will ultimately promote people to go beyond the inherent social scope and family restrictions when choosing a marriage partner. In addition, the increasing autonomy of women and the gradual reduction of family size also led to this phenomenon.
Married with cousins, children are more likely to have birth defects
One eighth of the genes of the first sibling are the same. Children born from marriage with the first representative brother and sister will get more of the same genes, which is likely to cause the expression of recessive harmful genes that are detrimental to survival and cause many disease attacks.
During random marriage, because the couple has no blood relationship and few identical genes, they carry different recessive disease genes, so it is not easy to form homozygotes of recessive disease genes.
When a close relative marries, it is very possible for the couple to carry the same recessive pathogenic gene, which is easy to meet in the offspring, thus increasing the incidence of genetic diseases in offspring.
It is estimated that 4% to 7% of the children born from the first sibling marriage may have congenital defects, while in random marriage, the probability is reduced to 3% to 4%.
Although this probability is not particularly high, if their next generation of children also marry their cousins of the same generation, a huge problem will arise, and the offspring will have more identical DNA, thus causing a higher probability of birth defects.
There are many examples of such tragedies in history, and this problem is still prevalent in some contemporary countries. For example, in Iceland, almost 330000 people in the country are concentrated in the capital Reykjavik. Many people often inadvertently marry close relatives. At present, local people choose to use mobile phone applications to prevent marriage with people with too many same genes.
The farther the kinship, the lower the risk
Marrying the first sibling may bring some risks to the offspring, but as the genetic gap between parents in each generation gradually widens, the possibility of marrying cousins to produce healthy offspring will also greatly increase. The second generation only accounted for 6.25% of the same genes between siblings, while the third generation only accounted for 3%. The average distance between spouses in modern America is the seventh representative parent, and there is no meaningful genetic relationship between two people.
Via：popsci.com
Former title: Go ahead, marry your cousin—it‘s not that bad for your future kids
Translator: Dong Dong
