* Age-related risk: The risk of having a child with Down syndrome increases significantly with the mother's age. This is because egg cells, unlike sperm cells, are arrested in their development from a young age. As a woman ages, her eggs accumulate more errors, including non-disjunction (the failure of chromosomes to separate properly during cell division), which can lead to an extra copy of chromosome 21.
* Meiotic Division: The process of egg development involves two meiotic divisions. Errors in the first meiotic division are more likely to result in an extra chromosome 21, and these errors are more common in older eggs.
* Sperm cell production: Sperm cells are constantly being produced throughout a man's life. This continuous production means there's less chance of accumulating errors like non-disjunction, compared to eggs that are arrested in development.
While it is true that sperm cells can also contribute to Down syndrome, it's much rarer. The increased risk associated with maternal age and the nature of egg development make it more likely that the extra chromosome 21 originates from the egg.
It's important to remember that this is a statistical trend, and not every case of Down syndrome is caused by an error in the egg cell. There are rare cases where the extra chromosome comes from the sperm, and even rarer cases where the origin cannot be determined.
