Question

Sol.(d) [Key Concept: Autosomal Non-disjunction — Down's Syndrome] DOWN'S SYNDROME is caused by autosomal PRIMARY non-disjunction — specifically non-disjunction of chromosome 21 (an autosome) during meiosis I (primary non-disjunction) or meiosis II. The resulting gamete has two copies of chromosome 21; fertilisation gives trisomy 21 (47 chromosomes). Klinefelter's (47,XXY) and Turner's (45,XO) syndromes are caused by non-disjunction of SEX chromosomes (not autosomes). Sickle cell anaemia is a single gene (point mutation) disorder, not a chromosome number disorder.