What evidence suggest that Down syndrome is more often the result of non-disjunction during oogenesis rather than during spermatogenesis?

Answer 1

One decent piece of evidence is that the incidence of newborns with Down's increases exponentially with the age of the mother. Once a woman reaches 45 years of age the chance of her having a child with Down's becomes 1:30, as opposed to 1:1667 if the woman is 20. No correlation like this has been found in the age of the father.

The increase in the incidence of Down's with mothers age is likely because all of the eggs a female has are with her from birth. They are trapped in one of the stages of the cell cycle (prophase 1 of meiosis). The longer the woman waits before becoming pregnant the older of an egg cell she is using. These older cells have been trapped in prophase 1 for many years. This, in some way, leads to an increase in the rate of non-dysjunction events (failure of 2 chromosomes to separate) and thus, an increase in trisomy 21.

Answer 2

Studies have shown that advanced maternal age is a major risk factor for Down syndrome, which is consistent with non-disjunction events happening more frequently during oogenesis. Additionally, the fact that almost all cases of paternal age effect are due to rare cases of genetic mutations, rather than non-disjunction events, also supports the idea that Down syndrome is more often linked to oogenesis.