Patient's question:
Dear Doctor: I have an important question to consult with you. My girlfriend's parents are first cousins who married (their mother was a pair of sisters). They have two children, with the older one being my girlfriend. They are both healthy, but I am concerned about whether our future descendants might be affected. Please help explain the knowledge related to this. I am a bit worried.Doctor's answer:
Here is the English translation of the provided text, with line breaks preserved as instructed:Marriage between individuals who are closely related by blood is known as consanguineous marriage. Close relatives are defined as two people who share a common ancestor within 3 to 4 generations. Consanguineous marriage can lead to genetic homozygosity, allowing deleterious recessive traits to be expressed or increasing the incidence of genetic diseases. This is because close relatives often share certain genes, for example, cousins have a 1/8 chance of carrying the same gene. The reasoning is as follows: It is known that a child inherits half of their genes from the father and the other half from the mother. Therefore, there is a 1/2 chance that a child shares the same gene with the father (or mother). E and F are cousins, and C (or D) shares a 1/2 chance of having the same gene with A. Thus, there is a 1/2 × 1/2 × 1/4 chance that C and D share the same gene (through A). Similarly, there is also a 1/4 chance that C and D share the same gene (through B). Therefore, the chance that C and D share the same gene (through both parents) is 1/4 + 1/4 = 1/2, meaning that siblings have a 1/2 chance of sharing the same gene. E shares a 1/2 chance of sharing the same gene with C, and F shares a 1/2 chance with D. Thus, the chance that E and F share the same gene (through C and D) is 1/2 × 1/2 × 1/2 = 1/8. Many human genetic diseases are controlled by recessive genes. Patients are homozygous for the recessive allele, and the general prevalence rate is low, but carriers of the disease-causing gene are relatively common. In random mating, the chance of both partners being carriers of the recessive disease gene is low, but in consanguineous marriage, the chance of two disease-causing genes encountering each other increases due to their shared genes. For example, in galactosemia, about 1 in 150 people in the general population are carriers of the recessive gene for this disease. In random mating, the probability of both partners being carriers is 1/150 × 1/150 = 1/22,500. The incidence of the disease in their offspring is 1/4, so the risk of the child developing the disease is 1/22,500 × 1/4 = 1/90,000. However, if cousins marry, the probability of them sharing the same recessive gene is 1/8, so the risk of their offspring developing the disease is 1/150 × 1/8 × 1/4 = 1/4,800, which is 19 times higher than in random mating (non-consanguineous marriage). If a family already has a case of recessive genetic disease, the danger of consanguineous marriage becomes even more apparent. For example, if a woman's uncle is a congenital deaf-mute patient, the danger of her marrying her first cousin can be determined through the following analysis: Since her uncle is a patient, it can be inferred that both her grandfather and grandmother are carriers of the disease-causing gene. Therefore, her father and aunt have a 2/3 chance of being carriers. The probability of them passing the disease-causing gene to their children is 1/2, so the woman and her cousin each have a 2/3 × 1/2 = 1/3 chance of being carriers. If they marry, the incidence of the disease in their offspring should be 1/3 × 1/3 × 1/4 = 1/36. However, if they each marry randomly, the incidence of the disease in their offspring would be 1/3 × 1/50 × 1/4 = 1/600 (assuming the probability of a carrier of the congenital deaf-mute disease-causing gene in the population is 1/50). In summary, consanguineous marriage increases the chance of harmful recessive genes encountering each other in both partners, significantly increasing the incidence of autosomal recessive genetic diseases in offspring, while also increasing the rates of miscarriage, stillbirths, early childhood mortality, and congenital deformities. Below is a comparison of the incidence of genetic diseases in two types of marriages: Additionally, the offspring of consanguineous marriage have four times the rate of intellectual disability and 3.8 times the rate of intellectual impairment compared to non-consanguineous marriage. According to statistics from the World Health Organization, about 8.1% of offspring from consanguineous marriage have genetic defects. This demonstrates the dangers of consanguineous marriage, and avoiding it is an effective measure to reduce the incidence of genetic diseases. For this reason, China's Marriage Law prohibits marriage between direct blood relatives and collateral relatives within three generations.