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.
Special population
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) and uncles (aunts). According to the survey, the rate of consanguineous marriage is about 0.7% in cities and 1.2% in rural areas. 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
Experts 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 Hitler, were married between cousins. 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.
Cause harm
legal provision
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.
dissenting views
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 AlanBittles, an internationally renowned anthropologist who specializes in consanguineous marriage, once praised: "China's marriage law is really a very good law, and the government legislates to care 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.
inherited disease
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 cousin and the second cousin that represents the intermarriage between the first cousin and the first cousin, the consanguinity index is 1/32, and so on for other types. Suppose the proportion of a genetic disease in the population is11000:
The risk of offspring of non-consanguineous marriage is11000000 (one in a million);
The risk of second cousin's married offspring is1128000.
The risk of offspring of first-degree cousin marriage is 1/32000.
The risk of offspring of brother and sister marriage is 1/8000.
Compared with non-consanguineous marriage, the risk of second-degree consanguineous marriage increases by 0.8 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.
relationship by blood
The following table lists the consanguineous coefficients of autosomal recessive genetic diseases in different levels of consanguineous marriage.
Inbreeding Types and 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. 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. 1980- 198 1 Beijing Han population survey shows that the inbreeding rate is 1%, and the average inbreeding coefficient is 0. 1%.
Ethnicity
Han people in China and ancient Korea have the principle that they should not marry with the same surname, and they should not marry men and women with paternal ancestors, but they should not marry men and women with matriarchal ancestors. Zhang, Empress Huidi of the Western Han Dynasty, is the daughter of Princess Tongmu Luyuan, and Chen Ye, Empress of Emperor Wudi of the Han Dynasty, is the daughter of Princess Guantao, her aunt. However, there are exceptions. For example, referring to the system of bone products, we know that the royal family of Silla will marry.
Catholicism forbids consanguineous marriage. However, royalty, nobility, etc. In Europe, because they often marry each other, marriage between non-close relatives becomes almost impossible, so they get special exemption from the church. But this special exemption later became an excuse for their divorce, and they were allowed to divorce because of the special consanguineous marriage in the church, which also prohibited divorce. Of the 50 states in the United States, 3 1 state declared it illegal to marry first cousins.
In ancient Egypt, although consanguineous marriage was not recognized, it was encouraged in some cases. This is related to maintaining the purity of the blood of the rulers of power. 20-50% of marriages in South Africa and Middle Eastern countries take place between close relatives. 10% of people are married to distant cousins or even closer relatives, or their parents are cousins.
FREQUENTLY ASKED QUESTIONS
In genetic counseling, the most common questions about consanguineous marriage mainly include the following three types:
1. How to judge which generation of close relatives get married?
Starting from the same generation, it is the first generation, and so on. If cousins get married, grandma belongs to the first generation, mother belongs to the second generation, and the combination of cousins is the third generation. China's marriage law stipulates that blood relatives are forbidden to get married within three generations, including the third generation, that is, cousins are forbidden to get married. In the Interpretation of the Marriage Law of the People's Republic of China, the original text is as follows: "(2) collateral blood relatives within three generations. Including: (1) brothers and sisters of both parents (including half brothers and sisters). That is, children of the same parents cannot get married. (2) uncles, uncles, aunts, aunts and nephews of different generations. Is that an uncle can't marry his brother's daughter; Aunt can't marry her brother's son; Uncle can't marry his sister's daughter; Menstruation can't marry my sister's son. " Only between parents and children belong to within three generations.
2. What is the risk of consanguineous marriage?
The coefficient of consanguineous marriage listed in the above table is its genetic risk. For diseases with low incidence, such as albinism, this risk rate is generally higher than that of random marriage; If it is a high-incidence disease, such as thalassemia or bean diseases in the south, the risk brought by this kind of consanguineous marriage is not necessarily higher than that brought by local people who marry at will.
3. Can the offspring of consanguineous marriage get married?
Although they are descendants of consanguineous marriage, they should not worry about the consequences of consanguineous marriage after marrying other non-consanguineous spouses if their families and themselves have no obvious history of genetic diseases. In other words, there is no obvious difference between the influence on offspring and normal marriage.
Japanese legend about "consanguineous marriage"
Legend has it that in a flood, everyone died, leaving only mother and son (or brother and sister). They get married and have children, which is the ancestor of human beings (or tribes). Such stories are widely circulated in the vast areas of Southeast Asia. This myth is also circulating in Japan and nearby South Korea and Okinawa. The legend of Sister Beidao also belongs to this category.
In the current edition of "Past and Present", it is recorded that the child of a farmer in Thailand fell asleep while sitting on a boat. At this time, the high tide washed the boat away. The boat drifted all the way to the island in the South China Sea, where they began to live, married when they grew up and became the ancestors of the island residents. The legend of Danpo circulating around Bazhang Island tells the story of mother and son and husband and wife. Legend has it that in Archean, people and livestock on the island were drowned by floods. Only one pregnant woman named Dana survived holding a tree. She lived in a cave by the sea and later gave birth to a boy. When I grew up, I had incest with my mother, gave birth to offspring and became the ancestors of the island residents. The legend of brother-sister marriage can also be heard in the myth of the origin of plug gods and the legend of Taoist ancestors everywhere. This legend may be in the same strain as the story of brother-sister marriage recorded in the legend of the Korean mountain god Huang Chengshen.
In the story of brother and sister intermarriage in Southeast Asia, it is said that the child born as a result of their close relatives' marriage was deformed at first, and then a normal child was born after consulting the gods for a special ceremony. This legend is the same as or similar to the myth of the founding of the People's Republic. Legends such as the old woman and mother marrying each other may be related to the belief of the world mother and child god.