Down syndrome is due to ?

Ans. is 'd' i.e., All of the above Cytogenetics of Down syndrome o Trisomy 21 (47, XX+21) 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 A7C/47XX + 21 (mosaicism has already been explained earlier).