It is said that there are many strange cases that paternity testers can encounter, and Deng Yajun, a paternity tester in Beijing, once shared a case he encountered in China News Weekly.
A couple encountered problems when registering their twin sons and needed a paternity test to settle down, and Deng Yajun gave them a paternity test. The results showed that only the eldest of the twins was the man's biological father. Deng Yajun said to the man: You are lucky, at least one of your sons is your own.
Although Deng Yajun said that the probability of twins being half-siblings is lower than winning the lottery, this is not the first time Deng Yajun has encountered such a case, she has encountered twice in her 17 years in the industry.
For this rare thing, some experts say that women need to have sex with different men in a very short period of time, is it really as simple as the experts say?
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Before introducing half-siblings, let's first understand how twins are formed.
Twins are divided into identical twins and fraternal twins. Identical twins are when the mother releases only one egg, and the egg encounters only one sperm and develops into a fertilized egg. But the fertilized egg forms two embryos when it splits in two early in development, and the two embryos continue to develop to form two babies.
From the perspective of genetic material, the genetic information of these two embryos comes from the same sperm and egg, so their genetic material is almost identical, so they must be of the same sex and look so similar that even their parents cannot distinguish them.
Since identical twins have identical genetic material, it is absolutely impossible for identical twins to be half-siblings.
Fraternal twins, as the name suggests, are twins developed from different fertilized eggs. We know that normally a woman's body only releases one egg per ovulation cycle, so women mostly only have one offspring at a time.
But there are also some women who are more special, they will release two eggs during the same ovulation period, theoretically when these two eggs encounter different sperm, they can be fertilized with sperm to develop into fertilized eggs.
Fraternal twins are also known as contemporaneous re-pregnancy, that is, women have a fertilized egg developing in the same ovulation cycle, but the body has released another egg, and the egg also forms a fertilized egg after encountering sperm, so one is pregnant first, and one is pregnant a few days after pregnancy, but they will be born at the same time.
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It is important to know that during the same ovulation period, a woman's two eggs are not released at the same time, but will be separated by a few days. In addition, the survival time of eggs is usually 24-48 hours, of which fertilization is the strongest in 15-18 hours, and sperm can survive in women for about 2-3 days.
That is, a woman who releases two eggs during the same ovulation period, and these two eggs meet the sperm of different men, will lead to the possibility that the woman is pregnant with half-siblings, which is more common in animals.
For example, cats, cats will mate with multiple males during estrus, and cats will release more than 2 eggs during the same ovulation period, which allows them to give birth to multiple kittens at once, and these kittens may have different fathers.
In addition to the simultaneous re-pregnancy, there is also a heterogeneous re-pregnancy, which is to be pregnant during the previous ovulation period, and then the next month still releases another egg, if it happens to be in the same room with the man at this time, the egg may also be fertilized.
But the probability of relapse is very low, because when a woman has been pregnant for a while, the body will not ovulate again.
The possibility that a woman can conceive two babies with different fathers in the same pregnancy cycle is rarely encountered in reality, first, because human morality constrains human sexual behavior; second, twins are rare, and fraternal twins are a less common type than twins; third, because few people do paternity testing, if all twins in the world do paternity testing, perhaps we will find that this kind of case is not rare in reality.
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Under natural conditions, the probability of half-siblings is very low, but with the help of modern medicine, half-siblings are not a rare thing, because because in countries where surrogacy is legal and homosexuality is legal, some male gays will use IVF, using eggs provided by the same woman, and then the surrogate mother will carry the twins. To put it more, surrogacy is not legal in our country, and no one can challenge this bottom line privately.
Although the method could theoretically produce half-siblings, many countries prohibit the artificial conception of twins because twins are very harmful to the mother.