In ancient times, many royal families and nobles chose to marry close relatives, especially cousins.
In modern society, Chinese law has prohibited marriage between close relatives, and we all know that it can lead to genetic diseases. However, there are still many cases of marriage between close relatives in some remote areas, which many people may have heard of.
What is consanguineous marriage? Consanguineous marriage means that two people who are related by blood are married. In addition, marriage within three generations of collateral relatives is also considered consanguineous marriage.
For example, if a man and a woman share the same grandparents, they are considered close relatives. Marriage between first cousins is the most common example of consanguineous marriage.
How did humans discover that close relatives should not marry?
In fact, the idea that close relatives should not marry was only gradually discovered. In ancient times, royalty and nobility would choose to marry close relatives in order to maintain the purity of their bloodlines. But later, people gradually realized that close relatives should not marry, mainly for two reasons.
Genetic determination : Since it is difficult to judge close relatives by observation, the Westermark effect is based on this research and concludes that two children who grew up together in childhood will not be sexually attracted to each other as adults, thus updating their understanding of close relatives .
Cohabitation time : Humans also use a second clue to judge close relatives, namely the length of time the two people have lived together. From the perspective of human evolutionary history, two blood relatives usually spend more time together because they share resources and help each other. More time spent together allows the older one to trigger the close relative recognition system.
The reason why people in ancient times chose to marry close relatives was twofold: firstly, the environment was harsh , and in many cases, two brothers would even marry the same wife; secondly, they were unaware of the dangers of marriage between close relatives .
However, as human evolution progressed, the "mysterious veil" of consanguineous marriage was lifted, and people discovered that consanguineous marriage has a very serious problem!
2. Children born to first cousins are more likely to have birth defects.
Many people have noticed that while China's marriage law prohibits marriage between close relatives, it is still legal in Japan. For Chinese people, marriage between close relatives is considered immoral, but in Japan, it is a common practice.
To ensure the purity of their bloodline , the Japanese Imperial Family relies on consanguineous marriage. They may marry not only cousins, but also siblings.
For example, in the mid-5th century, the eldest son of Emperor Ingyō of Japan, Kinashi Karu, married his younger sister, who was born of the same father and mother. This may have been the trend of the Japanese imperial family to have an unbroken imperial line.
In fact, in some remote areas of China, there is a similar situation to that of the Japanese Imperial Family, where people believe that marriage between cousins is a double blessing and a cause for celebration.
In some remote mountain villages known as "bachelor villages," local men, due to various reasons such as lack of economic means, are unable to marry and have children, so they resort to consanguineous marriage to reproduce. However, the harms of consanguineous marriage have gradually become apparent.
Emperor Meiji of Japan was an example of consanguineous marriage; 11 of his 15 children died young , while Emperor Taisho, who succeeded to the throne, was relatively healthy, but he also died not long after.
Offspring of consanguineous marriages have a higher probability of developing polygenic inherited diseases such as leukemia, intellectual disability, congenital malformations, anencephaly, and epilepsy, and there is a possibility of skipping generations in the inheritance .
In addition, in some remote areas of China, the probability of consanguineous marriage is higher due to underdeveloped transportation . However, because the probability of offspring having congenital genetic diseases is relatively high , people are gradually becoming aware of the dangers of consanguineous marriage.
Data shows that if siblings marry , their offspring have a 4% to 7% chance of having birth defects , while the probability of offspring from random marriages drops to 3% to 4%.
This is because unrelated couples rarely share the same genes, carry different recessive disease-causing genes, and have a significantly lower probability of developing recessive genetic diseases.
Studies have shown that offspring of consanguineous marriages are 150 times more likely to have recessive genetic diseases than offspring of random marriages , and their offspring also have a higher probability of early death.
3. The more distant the kinship, the lower the risk.
In fact, just as consanguineous marriage in humans has its drawbacks, consanguineous marriage in animals also has its drawbacks.
Canine historian David Hancock emphasizes that people's obsession with breeding purebred dogs is all about making them look better, but at the cost of the dogs' health . The Kennel Club in the UK even stipulates that purebred dogs can only mate with dogs of the same breed, leading to rampant inbreeding in dogs.
A BBC investigation has found that the 10,000 pugs in the UK are all offspring of 50 breeding dogs, resulting in health problems for these purebred dogs.
For example, Golden Retrievers are now prone to cancer, Dachshunds' bellies are almost touching the ground, and Shar Peis' faces are becoming increasingly flat, with more and more diseases appearing as they age. Even more alarming, long-term inbreeding can reduce a dog's reproductive ability !
In short, consanguineous marriage is unacceptable for both humans and dogs! From both a scientific and moral perspective, the disadvantages of consanguineous marriage outweigh the advantages.
References
[1] "Why can close relatives marry in Japan, but not in China? You'll learn something new after reading this." Eastday.com. 2018-10-22
[2] "Why might consanguineous marriage in humans result in defects? But in the animal kingdom, such as in cats and dogs, purebreds are often considered better?" NetEase Comments. 2018-07-17
[3] "Online Micro-Class | The Dangers of Consanguineous Marriage". Xinjiang Women's Voice. 2019-10-15
