An aneuploid state in which a third homologous chromosome is present in addition to the normal autosomal pair is called trisomy.
除了正常染色体对以外，还存在有第二种同源染色体的非整倍体状态，被称为三体性。
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Down syndrome (trisomy 21) is a disorder caused by the presence of an extra 21st chromosome.
唐氏综合症（21三体综合症）是由于患者额外多了一条21号染色体所致的疾病。
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Objective: a few trisomy 21 cells in normal chromosome karyotype was been observed for study of the correlation of clinical phenotype and development damage in sick children of Down's syndrome.