RE: Non-disjunction and Down's syndrome

Down's Syndrome can be caused by two different chromosomal abnormalities: nondisjunction of chromosome 21 (~95% of cases), or translocation in the chromosomes of either of the parents where, usually, part or all of chromosome 21 is attached to a larger chromosome.

Yes, the non-disjuction can occur in the father. My source indicates 20 percent of the cases arise from paternal non-disjunction. In these cases, the age of the mother is irrelevent. Also, I was taught that with very young fathers, this was more likely to happen, though I can't find a source for that now.

As for how the incidence increases with maternal age, human oogonia complete meiosis in the fetus (after fertilization), but are arrested in the diplotene stage of Prophase I of meiosis while still in the ovary. They remain in this stage of the cycle in until they are ovulated (or die).

In the diplotene stage, homologous chromosomes are paired and attached to each other at chiasmata. They have not yet lined up at the metaphase plate or separated from each other. Knowing this, it's easy to see how increased time in this stage (and hence increased exposure to mutagens, tetatogens, radiation, oxidixers, etc..) could lead to an increased frequency of non- disjunctions, translocations, or any other chromosomal abnormality.

I hope this helps. The following texts have more infomation:

Curtis, Helena, Biology, 4th Ed., 1983, Worth Publishers.

Gilbert, Scott F., Developmental Biology, 3rd Ed., 1991, Sinauer Assosiates.

Suzuki, David T., Anthony J. F. Griffiths, Jeffrey H. Miller, and Richard C. Lewontin, An Introduction to Genetic Analysis, 4th Ed., 1989, W. H. Freeman and Company.