Question

Sol.(c) [Key Concept: Down's Syndrome — Trisomy 21] Down's syndrome (trisomy 21) is caused by THREE COPIES OF CHROMOSOME 21 (total 47 chromosomes, 2n+1). Non-disjunction of chromosome 21 during meiosis produces a gamete with 2 copies of chromosome 21; when fertilised by a normal gamete, the zygote has 3 copies. Clinical features: intellectual disability, flat face, epicanthal folds, protruding tongue, short stature, and congenital heart defects. Two Y chromosomes = XYY syndrome; Three X chromosomes = XXX (superfemale); Monosomy = Turner syndrome (45,XO).