Chromosomal Disorders

Down's syndrome results from trisomy of chromosome 21, where an additional copy of this autosome gives the affected individual 47 chromosomes instead of the normal 46. This usually arises from non-disjunction during meiosis, when chromosome 21 fails to separate properly, producing a gamete with an extra chromosome. Characteristic features include a short stature, a small round head, a furrowed protruding tongue, and developmental delays. Turner's syndrome is incorrect because it is caused by monosomy of the X chromosome (45, X0), producing sterile females, not an extra autosome. Klinefelter's syndrome is wrong as it involves an additional X in males (47, XXY), a sex-chromosome aneuploidy rather than trisomy 21. Edward's syndrome arises from trisomy of chromosome 18, not 21, so although it is also an autosomal trisomy, the specific chromosome differs. As described in NCERT Class 12, Chapter 5 (Principles of Inheritance and Variation), chromosomal aneuploidies stem from non-disjunction events. As a verification step, the defining clue 'extra copy of chromosome 21' uniquely matches trisomy 21, confirming Down's syndrome as the answer.