A woman's eggs are . as old as she is. An embryo receives ½ it's chromosomes from Dad & ½ from Mom. DNA replicates in each parental chromosome pair & crosses over between 2 sister chromatids joined in the center that then split into egg cells. At 35 & >, cohesion of maternal sister chromatids is progressively impaired, ^ing the risk of an egg's receiving 2 or 0 copies of a chromosome. Her 2 + Dad's 1 Chr. 21 = DS.
Time = injury. A man gets a bit of a pass on defects since sperm renew constantly from stem cells.They are all young and used or lost before they get old.A woman's eggs are present at her birth.They are bombarded by background radiation, the woman's illnesses and diet,her good habits or bad. In the 20's the eggs have a low risk of DS but gradually increase to 1/20 by age 50.Accumulated damage is the theory.
Statistical risks. The risks of DS are at the low point in the early twenties, (~1/2700) and climb past 1/100 at 45 and over 1/20 after 55. The usual explanation is the accumulated toxic events of a woman's life can cause the eggs that sit in her ovary to become damaged as they age. Read more...