Patient's question:
I am 30 years old, and my boyfriend is 26 years old. However, his parents are first cousins who got married at a very young age, under 20. The boy and his two older sisters and one younger brother are all healthy. I want to ask the doctor, what is the probability that our child will be unhealthy if we get married and have a child? Thank you. Additional questionDoctor's answer:
Why do close relatives increase the incidence of genetic diseases in offspring? Because the inheritance of organisms is completed through genes transmitting information. Genes are the material basis of inheritance, passed down through germ cells to offspring, allowing parental traits to be expressed in the next generation. Each child has a 1/2 chance of sharing genes with their parents; all siblings have a 1/2 chance of sharing genes; grandparents, uncles, aunts, and nieces/nephews have a 1/4 chance of sharing genes; while first cousins have a 1/8 chance of sharing genes. For certain genetic diseases, the causative gene is recessive. If one partner carries this gene while the other does not, the disease gene can be masked, so the offspring does not develop the disease. Only when both partners carry the recessive gene will the offspring be affected. Therefore, consanguineous marriage increases the risk of recessive genetic diseases, such as albinism, congenital deafness and blindness; cerebellar malformation, phenylketonuria, galactosemia, etc.; it can also increase the incidence of polygenic genetic diseases, such as hydrocephalus, spina bifida, anencephaly, schizophrenia, congenital heart disease, epilepsy, etc. If the recessive gene is on the X chromosome, since females have two X chromosomes and may inherit a homozygous genotype from a common ancestor, while males are hemizygous and do not have a homozygous issue, consanguineous marriage has no effect on male offspring. Alternatively, if a disease-causing gene is confirmed to be X-linked recessive, marriage between first cousins or second cousins will not pass the disease to their offspring, but marriage between uncle-nephew or aunt-nephew cousins carries a higher risk of X-linked gene inheritance compared to autosomal inheritance. The impact of consanguineous marriage on offspring is primarily reflected in the increased incidence of recessive genetic diseases, as well as the risks of congenital malformations, premature birth, miscarriage, and infant mortality. When assessing the harm of consanguineous marriage to a population, it is usually necessary to calculate the average consanguinity coefficient (denoted as α) based on surveys of various consanguineous marriages. The larger the α value, the greater the harm to the population. Generally, an α value of 0.01 (i.e., 1%) is considered high. In developed and open societies, the α value is usually lower, while in, isolated, or socially isolated communities with special customs, the α value is higher. In 1980-1981, a survey of Han population in Beijing, China, showed a consanguineous marriage rate of 1.4% and an average consanguinity coefficient of 0.067%. How can one determine the generation of consanguineous marriage? Starting from the generation with a common ancestor, the first generation is counted, and so on. For example, if first cousins marry, their maternal grandmothers are the first generation, their mothers are the second generation, and the first cousins themselves are the third generation. China's Marriage Law prohibits marriage within three generations of kinship. How great is the risk of consanguineous marriage? The consanguinity coefficient is the genetic risk. For diseases with low incidence, such as albinism, this risk is generally higher than random marriage. However, for diseases with high incidence, such as thalassemia or favism in southern China, the risk of consanguineous marriage may not necessarily be higher than the risk of disease caused by random marriage in the local population. If one is the offspring of consanguineous marriage, can they marry someone with a close relative? Although they are the offspring of consanguineous marriage, if their family and the individual have no obvious genetic history, marrying a non-consanguineous partner should not be a concern about the consequences of consanguineous marriage. In other words, the impact on offspring is no different from normal marriage. Eugenics involves a wide range of aspects and is a complex issue with many factors, broadly including medical and social approaches. From a procedural standpoint, eugenics should "start from love," selecting a scientific partner, and individuals with severe genetic diseases or defects should not marry. Avoiding consanguineous marriage.