On the issue of consanguineous marriage

Close relatives (or kinship) refer to ancestors who have the same ancestry within three generations. If they marry, it is called consanguineous marriage. Couples who are closely related may get the same genes from their ancestors and pass them on to their children. If this gene is autosomal recessive, its children may get sick because they are homozygotes of the mutation. So inbreeding will increase the risk of some autosomal recessive genetic diseases. Inbreeding coefficient (IF) refers to the probability of obtaining a pair of homozygous or identical genes from offspring through inbreeding. Inbreeding: Couples who have the same ancestor in the generation below their great-grandfather are considered close relatives. 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. In the special population, the incidence of children born of consanguineous marriage is also significantly higher than that 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. The latest scientific point of view: Scientists in Western Australia, Australia are now challenging the problem that human inbreeding will give birth to unhealthy babies. This highly controversial topic has been puzzling people. In the past, people always thought that inbreeding would give birth to deformed babies, who suffered from many diseases, such as heart disease, mental retardation, deafness and even blindness. In 200 1 year, a scientific report published by an authoritative magazine in Australia claimed that the probability of a cousin getting married and giving birth to a deformed child was three times that of a non-relative. However, Allen Bito, a professor of comparative genomics at Murdoch University, has spent 30 years studying this topic and found that most of her cousins are healthy. In Western Australia, Australia, about 500 couples get married through cousins. Professor Allen came to the conclusion through research: "In western culture, there is a general idea that cousin marriage leads to genetic tragedy, but a large number of facts have proved this idea wrong. Cousins can also give birth to healthy babies. " Professor Bito conducted a survey in 1 1 countries, and found that the probability of giving birth to a deformed baby was 2%, while the probability of giving birth to a deformed baby by a close relative only increased to 4%. He also found that the infant mortality rate of close relatives is only 1.2%, which coincides with his research in 2002: the probability of a cousin getting married and giving birth to a deformed baby is less than 3%. This question triggered a heated medical debate at the medical seminar of the Royal Society of East London. Some researchers and politicians claim that in Britain, consanguineous marriage can lead to very serious consequences, and their children may be deformed or even die prematurely. The opponents represented by Professor Bito spoke: "As for the genetic tragedy of marriage, I think it is more related to excessive drinking and smoking." Many famous people, such as Einstein, Darwin and Jerry Lee Lewis, a rock musician, were married by close relatives. Cousin marriage is common in some countries, such as Pakistan, South Asia and Middle East countries. In this debate, a doctor agreed with Professor Bito very much. He claims that there is only a 4% chance that cousins will have baby defects when they get married, which is nothing in genetics, so don't worry about genetic problems. Professor Bito said that it is wrong to say that cousin marriage is extremely rare. In fact, in many parts of the earth, cousins get married more frequently. The population of these areas adds up to more than 654.38 billion, and about 20% to 50% of marriages there are related by blood. Professor Bito believes that as more and more immigrants enter Australia, more and more cousins will get married. In view of the large number of marriages between cousins, Bibitoggio called for further medical research on this issue in order to better explain this mystery. 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 marriage pattern: if there is a gap between the first cousin and the second cousin that represents the intermarriage between the first cousin and the first cousin, then the consanguinity index is 1/32 = 0.03 125, and so on for other types. Suppose that the proportion of a genetic disease in the population is11000; the risk of non-inbreeding offspring is1/500x1/500x1/4 =1/kloc-0. The offspring risk of second cousin marriage is1/500×1/64×1/4 =1128000; The risk of offspring married by close relatives is 1/500× 1/60. The risk of the offspring of 32,000 sibling marriages is1/500×1/4×1/4 =1/8000. Compared with consanguineous marriage, the risk of second-degree relatives is 7.8 times higher. 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 consanguineous table listed in the following table is the consanguineous coefficient of autosomal recessive genetic diseases in different grades of consanguineous marriages. Type of consanguineous marriage and its consanguineous marriage coefficient (f) between first-degree parents and children 1/4 between brothers and sisters 1/4 between half-brothers and sisters 1/ 8 between uncles 1/8 between uncles. 8 Third Cousins116 Third Cousins116 Half Cousins 1/32 Second Cousins (from Cousins) 1/64 If it is a recessive gene on the X chromosome, it is because women. If it is determined that a certain pathogenic gene belongs to X-linked recessive inheritance, cousin's marriage will not inherit the disease. The influence of inbreeding 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%. The harm of consanguineous marriage The topic of consanguineous marriage has been one of the most contents since the establishment of genetic counseling. Article 6 of Chapter 1 of the Marriage Law passed at the Third Session of the Fifth National People's Congress 1980 clearly stipulates that lineal blood relatives and collateral blood relatives within three generations are defined as blood relatives, and marriage is prohibited. Marriage law is one of the national laws, and every citizen must abide by it. Objection Some people envy the freedom abroad and say, "It doesn't matter whether you are a close relative or not, you can go to 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.