A newborn screening flags Down syndrome — the extra chromosome 21 came from nondisjunction in the mother's egg, risk rising from 1/1500 at age 20 to 1/100 at 45. Turner girls (XO) grow up short with undeveloped ovaries — but with growth hormone and estrogen therapy they live full lives. Klinefelter boys (XXY) are often diagnosed only in infertility clinics as adults.