<prompt> Down syndrome is due to ?Trisomy 21Translocation 13-1521Translocation 2221All of the above  <response>: Ans. is d i.e., All of the above Cytogenetics of Down syndrome o Trisomy 21 47, XX21 is the most common 95 chromosomal abnormality in Down syndrome. o In 3 of cases extra chromosomal material is derived from the presence of a Robeson translocation of long arm of chromosome 21 to another acrocentric chromosome 22 or 14 or 15. o In Robeson translocation, the translocation occurs between two acrocentric chromosomes. Breaks occur close to the centromere of each chromosome --- large fragments of both chromoses fuse together and small fragments together formation of one very large chromosome and one very small which is last later. In Down syndrome large fragment of 14 or 15 or 22 chromosome fuses with large fragment of chromosome 21 --- Extra material comes on 21 and it acts like third 21st chromosome -- Trisomy 21. o In 2 of Down syndrome patients, there is mosaicism, i.e. occurance of two or more different types of populations in the same individual -- 4 A7C47XX 21 mosaicism has already been explained earlier. 