Objective:To probe the possibility of using STR polymorphism on chromosome 21 to diagnose Down’s syndrome.At the same time

some parents of Down’s syndrome were analysed using the same motbed.To develop a rapid anti accurate method for Down’s syndrome diagnosis and review its pattern.Methods:The small tandem repeats(STR)of D21S11 and D21S1414 on chromosome 21 were analysized by polymerase chain reaction(PCR).100 Down’s syndrome samples(blood)and 20 parents’ samples(blood)were analyzed with.denaturing polyacrylamide gel(8%)and Silver Nitrate staining.Results:(1) The normal person and two kinds of status in the genotype of a STR gene:①Two bands of same size on the gel electrophoresis were displayed to the heterozygote of two different alleles.②A band on gel electrophoresis was shown to the homozygote of two same allele But the width and thickness of the band were about two times of a single one.(2)There were three kinds of characteristic changes of the number and/or the bands thickness of allele for the Down’s Syndrome:①Three different alleles

with three bands of same thickness on gel electrophoresis.②Only two of the three alleles are the same

two bands on gel electrophoresis

and one of the two bands was two times thicker than the other.③Three alleles were the same

only one band on gel electrophoresis.The width and thickness of the band are about three times than single band of normal.(3) The analysis about parents of 20 cases Down’s syndrome shows that the two bands were usually derived from mother

and another derived from father in bands of the Down’s syndrome on gel electrophoresis.Conclusions:(1)As genetic marker with STR sites (D21S11 and D21S1414)of chromosome 21

Down’s syndrome can be diagnosed by PCR

denaturing polyacrylamide gel and Silver nitrate staining analysis.(2)Genetic analysis of the parents of Down’s syndrome show that onset of Down’s syndrome does not have relationship with the karyotype of the parents.Besides

the extra chromosome 21 is derived from mother usually.