Why does the extra genetic material of chromosome 21 lead to Down syndrome?
Down syndrome illustrates the principle of gene dosage, which states that the amount of a gene product produced per cell is proportionate to the number of copies of that gene present. In other words, the amount of protein produced by all or nearly all genes that lie on chromosome 21 is 150% of normal in trisomy 21 cells. There may be a critical region of chromosome 21, which, when present in triplicate, is both necessary and sufficient to produce the phenotypic features of Down syndrome.
Development of Leukemia