Why is there a scientific basis for prohibiting consanguineous marriage?

In biology, the risk of genetic diseases in offspring born by close relatives is greatly increased. In ancient China, the traditional idea was against consanguineous marriage, and now it is also prohibited by law to marry within three generations.

Three generations of close relatives

Three generations means that you (and your brothers and sisters) are the first generation, your parents (and your parents' brothers and sisters) are the second generation, and your grandparents (and their brothers and sisters) are the third generation.

Three-generation close relatives refer to relatives who have the same blood relationship within three generations, including collateral blood relatives within three generations, such as the children of your mother's sister. You belong to three generations of close relatives. According to the truth, you can't get married.

human gene

Our human somatic cells are diploid, that is, they have two complete sets of chromosomes, ***23 pairs, 46 chromosomes, including about 39,000 genes, consisting of about 365,438+600 million base pairs.

In biology, each gene has a unique function. For example, human beings have genes that determine single eyelid and double eyelid, genes that determine skin color, genes that determine height, and genes that affect IQ.

Because human beings have two sets of chromosomes, and each set of chromosomes comes from father and mother respectively, the alleles in the two sets of chromosomes may be different; For example, if one set of chromosomes represents a single eyelid and the other set represents a double eyelid, then there will be a selection problem. You can't let one eye have a single eyelid and the other eye have a double eyelid!

In terms of gene expression, organisms will choose two groups of genes. One group of genes takes precedence over another group, which we call dominant genes (represented by capital letters), and the other group of genes that are not expressed are recessive genes (represented by lowercase letters). For example, if the double eyelid is the dominant gene A and the single eyelid is A, there will be three situations:

AA: Expressed gene A, characterized by double eyelids.

Aa: Expressed gene A, characterized by double eyelids.

Aa: Expression of recessive gene A, characterized by single eyelid.

Of the 39,000 human genes, all the other genes have corresponding alleles, except for a few that have no alleles on the sex chromosome. After millions of years of evolution, the diversity of human genome is very rich.

After long-term natural selection, many genes that are not conducive to human beings have evolved into recessive genes, and some genes even have symptoms, such as albinism, red-green blindness, hemophilia, cheap cell anemia and so on. At present, there are more than 200 known autosomal recessive genetic diseases/kloc-0.

The risk of consanguineous childbearing

Under normal circumstances, everyone will carry hundreds of recessive genetic disease genes, because they are covered by dominant genes and will not show symptoms. These recessive genetic disease genes will be inherited in the family. From the probability point of view, the closer the blood relationship, the more genes in the same recessive genetic disease.

If you marry someone who is not related by blood, the probability of two people carrying the same recessive genetic disease gene is very low. If the offspring have been intermarriing with outsiders, some recessive genetic diseases in the family will eventually disappear.

If it is inbreeding, the probability of offspring inheriting two recessive genetic diseases will be greatly increased, and offspring will easily cause some rare family genetic diseases, especially affecting the IQ of offspring, and even congenital malformation, so the state explicitly prohibits inbreeding.

situation

Hemophilia, also known as "royal disease", is a hereditary coagulation disorder. The gene on X chromosome is characterized by insufficient active thrombin and long clotting time. Even minor trauma can lead to severe bleeding.

It is also called the royal disease, because this genetic disease is widely spread among the royal families in Europe. In the past, in order to ensure the purity of European royal families, intermarriage between royal families was very frequent, and even many close relatives intermarried within three generations, which led to many genetic diseases in the whole royal family, and even some offspring showed symptoms such as strange appearance, physical deformity and low IQ.