Non dysjunction. This term means that one of the earliest divisions of the embryo after conception delivers 3 number 5 chromosomes to a daughter cell and only one to the other daughter cell. The trisomy cell survives to create down's syndrome babies.
The way things are. The male contribution to conception comes from a stem cell system continuously producing new sperm from puberty until ??The healthiest sperm are the most likely to complete the process. (defective ones don't swim as well) Women usually release one egg at a time, that represents accumulated age/toxic exposures/etc.If an extra 21 chromosome is in an egg, it produces a defective but survivable embryo.
Selection variables. If a woman generates an egg that contains 2 rather than 1 chromosome 21's, the addition of a 21 from the sperm will cause the trisomy 21 condition. Some of these miscarry & some deliver. When males generate a sperm that carries irregular information, there is speculation that the ones with bad information do not swim as well & thus can't pass the defect. Read more...