Abstract: Objective: To investigate the genetic polymorphisms of 3 short tandem repeat (STR) loci D21S11，D21S1440 and Penta D on chromosome 21 in Tianjin Han population. The aim of the study was to provide basic data for the use of 3 STR loci in gene diagnosis and prenatal gene diagnosis of Down syndrome (DS). Methods: Blood samples from 332 unrelated individuals of Tianjin Han population were collected. QF-PCR and ABI PRISM 377 sequence were used. The frequencies of the genotypes were tested with Hardy-Weinberg equilibrium. Genetic analysis was performed to conclude some data of population genetics such as the frequency of the alleles，the heterozygosity of observation(Ho)，the polymorphism information content(PIC)，the probability of discrimination power(DP)，and the probability of exclusion(PE). Results: 6，4，8 alleles of D21S11，D21S1440 and Penta D were observed respectively. The frequencies of the genotypes were in good agreement with Hardy-Weinberg equilibrium. The Ho of 3 STR loci were 0.617, 0.705, 0.867. The PIC were 0.755, 0.596, 0.795. The DP were 0.916, 0.794, 0.931. The PE were 0.312, 0.436, 0.730. Conclusion: D21S11，D21S1440 and Penta D STR loci had high heterozygosity in Tianjin Han population. They were good genetic markers of chromosome 21 and can be used in gene diagnosis and prenatal gene diagnosis of DS.

Key words: tandem repeat sequences, down syndrome, polymorphism, genetic, polymerase chain reaction, TIANJIN