The incidence of trisomy 21 (also known as Down syndrome) in newborns is 1 in 700, and without prenatal screening, these children usually survive to 20 or 30 years of age if you don’t give them special help. Like chromosomal disorders, we say, we have 46 chromosomes, 23 pairs. So of these 23 pairs of chromosomes, it is theoretically possible to form trisomies or tetrasomies. The first chromosome, for example, it could also form trisomies, but many chromosomes, if the abnormality is too large, usually miscarriage occurs early on. It is a natural selection and does not survive at all. Then, for example, chromosome 18 and chromosome 13, which can also be trisomic. But the incidence of this child is relatively low, it survives to this stage of birth, it’s rare, about one in 1500 children. It usually dies between the ages of three months and half a year. Although it causes great pain to the mother, the overall harm is not great. But Trisomy 21 can survive into the 30s, and that becomes a problem. The average IQ is very low, only between 30 and 50, and the person survives with a very low IQ, so it adds a great burden to society and to the family. So it is not that other chromosomal abnormalities are not serious, just that they are naturally eliminated. Trisomy 21, or Down syndrome (DS), also known as congenital dysmorphism (OMIM#190685) is a disease caused by structural aberrations of the chromosomes, with the patient having a karyotype of 47,XX(XY),+21 resulting from the non-separation of chromosome 21 during meiosis and the formation of an abnormal egg. 20 years) or younger, and is currently reported to be associated with older fathers as well. The general characteristics of DS: 1. syndrome 2. most cases are new and sporadic 3. identical twins are consistent 4. male patients are not fertile, while very few female patients are fertile 5. the incidence of the disease increases with the age of the mother, especially when older than 35 years 6. patients have a short life expectancy and the brain shows amyloid plaques when the patient reaches middle age, with Alzheimer’s disease (i.e., dementia), with dementia symptoms; immune deficiency, congenital heart disease is also increased, treatment with antibiotics and cardiac surgery can prolong the life of patients 7, the phenotypic characteristics of the degree of expression of different 8, acute leukemia mortality increased by 20 times.