A total of 15 blood samples were investigated in this study by both conventional cytogenetic tehnique and QF-PCR method. Analysis of fluorescent amplified DNA samples is made qualitatively by determination of allelic signal number and quantitatively by measurement of allelic fluorescent intensity. Normal heterozygous samples display diallelic peaks corresponding to two different alleles with peak fluorescence intensity ratio of approximately 1:1, whereas normal homozygous samples display only one peak corresponding of two identical alleles (Fig. 1). Trisomic samples display either 3 peaks of similar intensity or 2 peaks but with fluorescence intensity ratio of approx. 2:1 [3].

All 5 normal individuals examined in our study was heterozygous and showed two allelic peaks for both D21S11 and Penta D STR markers with peak height ratio of 1:1. Four of ten 21 trisomic samples showed a triallelic pattern with peak height ratio of approximately 1:1:1 for both D21S11 and Penta D markers. Four trisomic samples showed a diallelic pattern with peak height ratio of 1.6 - 2.3 for D21S11 marker and 1.7 – 1.9 for  Penta D marker. One trisomic sample displayed a triallelic pattern for D21S11 marker and a diallelic pattern with peak height ratio of 2.3 for Penta D marker whereas another trisomic sample showed a triallelic pattern for Penta D marker and a diallelic pattern with peak height ratio of 1.7 for D21S11 marker.  Figure 2 (A) illustrates a trisomy 21 case with a triallelic pattern for both D21S11 and Penta D markers. Figure 2 (B) shows a trisomy 21 case with a diallelic pattern (2:1 dosage ratio) for both 21 chromosome markers.

The results obtained using quantitative fluorescent PCR were in concordance with the conventional cytogenetic results. 

This study represent our first attempt to evaluate a molecular method for detection of trisomy 21. The preliminary data obtained successfully confirm the cytogenetic diagnosis of trisomy 21 and demonstrated that this tehnique is suitable for the rapid postnatal detection of Down syndrome.