Inbreeding. A couple are close relatives if they have the same ancestor in the next generation of their great-grandfather. Cousin marriage is a relatively common consanguineous marriage. Most countries do not encourage consanguineous marriage, or even prohibit it. When close relatives get married, the mortality rate of offspring is high, and dementia, deformed children and genetic diseases often appear. This is because couples who get married by close relatives get more of the same genes from their ancestors, which makes it easy for recessive harmful genes that are not conducive to survival to meet in their offspring (that is, homozygous), so it is easy to give birth to children with poor quality. According to the estimation of the World Health Organization, everyone in the population carries about 5-6 recessive genetic diseases. In random marriage (non-consanguineous marriage), it is not easy to form homozygotes (patients) with recessive pathogenic genes because the husband and wife are not related by blood, have few identical genes and carry different recessive pathogenic genes. However, when close relatives get married, it is very likely that both husband and wife carry the same recessive pathogenic gene, so it is easy to meet in offspring, which will increase the incidence of genetic diseases in offspring. The following table lists several recessive genetic diseases, indicating that the incidence rate of offspring of consanguineous marriage is several times that of offspring of non-consanguineous marriage.
In addition, the incidence of polygenic genetic diseases is high, such as hypertension, schizophrenia, congenital heart disease, anencephaly, epilepsy and so on. The mortality rate of children born of consanguineous marriage is also significantly higher than that of children born of non-consanguineous marriage. To this end, many countries have passed laws prohibiting consanguineous marriage. Article 6 of China's Marriage Law stipulates: "lineal blood relatives and collateral blood relatives within three generations are forbidden to get married." Lineal consanguinity refers to the relationship between parents and children, grandparents and grandchildren, and grandparents and grandchildren. Collateral blood relatives within three generations include compatriots, uncles (aunts) nephews (female), uncles (aunts) nephews (female) and cousins. According to the survey, the rate of consanguineous marriage is about 0.7% in cities, 1.2% in rural areas, and some are as high as 2.8%. In some mountainous areas, rural areas and islands, due to underdeveloped transportation, the proportion of close relatives is high, so there are more genetic diseases. Therefore, forbidding consanguineous marriage is an effective measure to reduce or even eliminate the incidence of recessive genetic diseases and improve the quality of the population, which is one of the main contents of eugenics. [Edit this paragraph] Why not advocate consanguineous marriage?
Inbreeding is one of the most important contents of genetic counseling since its establishment. Article 6 of Chapter 1 of the Marriage Law passed by the Third Session of the Fifth National People's Congress in September 1980 clearly stipulates that it is forbidden to marry lineal blood relatives and collateral blood relatives within three generations. Marriage law is one of the national laws, and every citizen must abide by it.
Some people envy freedom abroad and say, "It doesn't matter whether you are a close relative or not, you can go to the church for a wedding if you want …" He can't be regarded as having a clear understanding of the meaning of freedom. Professor Allen Beattre, an internationally renowned anthropologist who specializes in consanguineous marriage, once praised: "China's marriage law is really a very good law, and the government legislation cares about the quality of the population. Unfortunately, there is no such law in Britain. "
Others said: "Our family has never had a genetic disease for generations, and I firmly believe that we will never get a genetic disease at all." But after all, the questioner is still uncertain and wants to ask how to detect genetic diseases, so the next question is "We are willing to spend money on testing and ask you which hospital is the best for us to test?" In this regard, there is no need to talk about marriage law. Let's use scientific calculation to explain why inbreeding is not advocated.
According to experts' estimation, every normal person may carry several or even a dozen harmful recessive alleles, and inbreeding will make these recessive alleles have more opportunities to meet and produce genetic abnormalities. Half of the human nuclear genome comes from father and half from mother. In the case of consanguineous marriage, the probability of combining two equally problematic genes is far greater than that of non-consanguineous marriage. What is the risk of inbreeding? Let's calculate from the following mating patterns:
If there is a gap between the first representative and the second representative, which represents the intermarriage between the first representative and the second representative, the inbreeding index is 1/32 = 0.05438+025, and so on for other types. Suppose the proportion of a genetic disease in the population is11000:
The offspring risk of non-consanguineous marriage is1/500x1/500x1/4 =11000000 (one in a million);
The offspring risk of second cousin marriage is1/500×1/64×1/4 =1128000.
The risk of offspring married by close relatives is1/500×116×1/4 =1/32000.
The offspring risk of sibling marriage is1/500×1/4×1/4 =1/8000.
Compared with non-consanguineous marriage, the risk of second-degree relatives increased by 7.8 times; The risk of first-degree relatives increased by 3 1 times; The risk rate of sibling marriage is 125 times that of normal random marriage!
If we follow the expert's advice, everyone may carry several or even 10 recessive harmful genes, and the risk of inbreeding offspring will be higher. For example, glycogen storage disease, an autosomal recessive genetic disease, has many possible types, and its gene is composed of 17 exons. The married offspring of people with mutations in different exons will not be abnormal, but the pairing of two identical exons is usually caused by inbreeding.
Some people say that intermarriage between different ethnic groups will increase the incidence of cancer, which is totally unfounded. Modern humans have no reproductive isolation and no species distinction. White people, black people and yellow people are all the same species, and their blood relationship is very close. For example, the molecular differences between blacks and whites may be smaller than the genetic differences between Beijingers and Shanghainese. This is a genetic definition, that is, the differences between individuals within a family may be greater than the differences between individuals within a family.
Others take some ethnic minorities in our country as examples, saying that consanguineous marriage there makes everyone there beautiful, with double eyelids and big eyes, and so on, and to demonstrate the benefits of consanguineous marriage. This idea is natural, but this kind of propaganda is absurd and irresponsible. The characteristics of double eyelids are an important genetic feature. Because most ethnic minorities in the south have this genetic feature, it will be shown whether relatives are married or not, and it has nothing to do with the marriage of relatives.
The following table lists the consanguineous coefficients of autosomal recessive genetic diseases in different levels of consanguineous marriage.
Types of consanguineous marriage and its consanguineous marriage coefficient
Inbreeding type Inbreeding level Inbreeding coefficient (F)
The first level between parents and children 1/4
Grade between brothers and sisters 1/4
Half-brother or half-brother, level 2 1/8
Between an uncle and a daughter level 2 1/8
Between uncles (or between uncles) Grade II 1/8
Cousin level 3116
Between Cousins Level 3116
Between half-cousins, level 4 1/32
Second Cousin (from Cousin) Level 5 1/64
If it is a recessive gene on the X chromosome, because women have two X chromosomes, they may get homozygous genotypes from the same ancestor, while men are hemizygotes, so there is no homozygous problem, so inbreeding has no effect on the boys born. In other words, if it is determined that a pathogenic gene belongs to X-linked recessive inheritance, cousins will not pass the disease on to their children when they get married, but the genetic risk of X-linked genes after cousins get married is greater than that of autosomes.
The influence of consanguineous marriage on offspring is mainly manifested in increasing the incidence of recessive genetic diseases, and the risk of congenital malformation, premature abortion and premature death is also increased. When evaluating the harm of consanguineous marriage to people, it is usually necessary to calculate the average consanguineous marriage coefficient (expressed by a value) on the basis of investigating all kinds of consanguineous marriages. The greater the value of a, the greater the harm to people. Generally, the value of a is 0.0 1 (that is, 1%) as a high value. Generally, in a developed and open society, the value of A is low, while in a closed, isolated or special society, the value of A is high. From 1980 to 198 1, the survey of Han population in Beijing shows that the inbreeding rate is 1.4%, and the average inbreeding coefficient is 0.067%.
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