Context: Down Syndrome or Trisomy 21, with three sets of chromosome number 21 is the commonest chromosomal abnormality in newborn. There are three types of Trisomy 21: Free Trisomy 21, Translocation Trisomy 21 and Mosaic Trisomy 21. Aims: The study aimed at finding the frequency of Down syndrome and its various cytogenetic types in a population from North East India. Methods and Materials: Karyotyping from G-Banded peripheral lymphocyte of patients with suspected chromosomal abnormality was done from peripheral blood and stained with Giemsa stain as per the Standard Operating Protocol of the Diagnostic Genetic Laboratory (CFDMGD). One to three ml of blood was withdrawn aseptically from each patient. 20-30 spreads were analyzed for each case. For mosaics, 30-50 spreads were studied. The slides were analysed for detection of various chromosomal abnormalities including Down syndrome (Trisomy 21). For the translocation Down Syndrome cases, parents were investigated to determine the parental carrier status. Results: 38 cases of Trisomy 21 were detected. Free Trisomy 21 was found in 92.11% cases, translocation trisomy 21 was seen in 2.63% case and Mosaic Trisomy 21 was seen in 5.26% cases. Male: female ratio was 1.38:1. Conclusions: Knowledge of the cytogenetic types has important clinical implications as it helps clinician/geneticist determine the recurrence risk in subsequent pregnancies and helps couples take an informed decision. This in turn would help in decreasing the load of the disease in society.
Keywords: Down syndrome, Trisomy 21, Free trisomy 21, Translocation trisomy 21, Mosaic, North East India