Caused by the abnormality of chromosome 21, Down syndrome is the most common chromosomal disease as well as one of the most frequent known genetic causes of mental retardation.
唐氏综合症由人体21号染色体病变引起。它是最常见的染色体疾病，也是最主要的导致智力落后的基因病变。
2
Klinefelter syndrome is still the most common chromosomal disease, balanced translocation accounts for largest proportion of autosomal abnormality.