The STR Polymorphism On Chromosome 21 And The Diagnosis Of Down's Syndrome

ObjectiveTo proble application of STR polymorphism on chromosome 21, the small tandem repeats (STR) of D21S11 and D21S1414 on chromosome 21 were analysized by polymerase chain reaction (PCR), denaturing polyacrylamide gel and Silver Nitrate staining. At the same time, the some parents of Down's syndrome were analysed by the method. It aims at developing a rapid and accurate method for Down's syndrome diagnosis and understanding genetic pattern.Methods1. PCR was performed with DNA template from 100 Down's syndrome samples ( blood) and 20 parents samples (blood) by the specific primers for D21S11 and D21S1414.2. PCR products for D21S11 and D21S1414 were subjected to denaturing polyacrylamide gel (8%) electrophoresis, followed sliver nitrate staining, and gel image analysis.Results1.The normal person has two kinds of status in the genotype of a STR gene:â‘ the heterozygote of two different allele show the two bands of same size on the gel electrophoresis.â‘¡the homozygote of two same allele show a band on the gel electrophoresis, But the width and thickness of the band are about two times of a single one.2. It takes three kinds of characteristic changes of the number and/or the bands thickness of allele for the Down's Syndrome:â‘ If they are three different alleles, it will have three bands of same thickness on gel electrophoresis.â‘¡If two of the three alleles are the same, there will be two bands on gel electrophoresis, and one band is two times thicker than the other.â‘¢if the three alleles are the same, there will be only one band on gel electrophoresis. Width and thickness of the band are about three times than single band of normal control.3. The analysis about parents of 20 cases Down's syndrome shows that the two bands derived from mother usually, and another derived from father in bands of the Down's syndrome on gel electrophoresis. Conclusions1. 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. The parents of Down's syndrome show that onset of Down's syndrome has not relationship with the karyotype of the parents. It is due to nondisjunction of meiotic division. Besides, the extra chromosome 21 derived from mother usually.