Down syndrome, or trisomy 21, also known as congenital stupidity, is a cytogenetic chromosomal disorder that is mostly not inherited. Whether Down syndrome is inherited to the next generation is correlated with the patient’s karyotype.1. Complete Trisomy 21: The probability of occurrence accounts for 95% of clinical cases. It is caused by an abnormal meiosis during the development of sperm cells or egg cells, resulting in an extra chromosome 21, which is not inherited. 2. Chimeric Trisomy 21: The probability of occurrence is very low, and the occurrence of this karyotype is only about 1%. Patients with two or even more cell lines have different clinical manifestations depending on the percentage of abnormal cells. Some patients have a very small percentage of abnormal cells or even a completely normal phenotype, but their children may acquire additional chromosomes and cause the disease; 3. Translocation type 21-trisomy: Patients with translocation type 21-trisomy have an unbalanced translocation in their cells, and any parent with this translocation type may pass it on to the next generation. If a child is screened for Down syndrome, karyotype testing is very important to help clarify the karyotype of the child and the type of Down syndrome, and to clarify whether it is a hereditary disease.