Under normal circumstances, each human cell has 23 pairs of chromosomes. One chromosome in each pair comes from the father, and one from the mother. When the body makes egg cells or sperm cells, the process of meiosis separates the pairs, so that each reproductive cell has just 23 single chromosomes.
The majority (over 90%) of cases of Down Syndrome are the result of an error in meiosis which results in an egg cell with two copies of chromosome 21. If this abnormal egg cell is fertilized, the cells of the resulting embryo have three copies of that chromosome, two from the mother and one from the father. This results in a situation known as Trisomy 21. Because it results from a one-time mutation of an egg cell, Trisomy 21 cannot be inherited.
Another form of Down Syndrome is Translocation Down Syndrome, a condition in which the individual has the normal number of chromosomes, but has part or all of an extra chromosome 21 attached onto the end of some other chromosome, making it much longer than normal. This condition is rare, causing only about 4% of all cases of Down Syndrome, but it can indeed be inherited; because there are so few cases, no one knows what the exact chances are. It is estimated that fewer than 1% of all Down Syndrome cases are inherited.
That would depend on the genotype of the person they married. Down syndrome is a recessive-linked chromosomal disorder that is expressed when a person inherits the recessive allele for Down Syndrome from both parents. That person's genotype would be rr, since they have both recessive alleles.
If the person they marry is not a carrier of Down Syndrome, having a genotype of RR, all their children will not have the disease, as this genotypic cross produces offspring with the Rr genotype. Four out of four offspring (100%) will have this genotype, meaning they are carriers of the disease, but will not have the disease themselves.
On the other hand, if the person with Down Syndrome marries a person who is a carrier of the disease, that genotype would be Rr. This genotypic cross will produce two out of four offspring (50%) without the disease, but carriers of the disease (Rr), while the other two out of four (50%) offspring would be afflicted with the disease, as they have the recessive allele combination (rr). So it depends on the genotype of the person they marry.
BandmanJoe - no, that is not correct.
A person with DS has 3 copies of the 21srt chromosome. During meiosis a set from one parent does not separate properly and they get two genes from one parent and one from the other. In the most common form, it occurs in every cell in the body (in a rarer form, it happens after the zygote is formed and only occurs in some portion of the person's cells).
Because of this, if they had a child, the person with DS (if fertile, many are not fertile) would have a 50% chance of passing 2 of his/her chromosomes off and having a child with DS. Otherwise, the other 50% of the time they would have the 1/800 chance that all of us have.
More important than the science, though, is understanding the implications. People (including the medical community) see this as an adverse outcome - it is something bad to be avoided. On the other hand, most have little or no experience with people with DS and their assumptions that it is "bad" is mostly due to lack of familiarity. People who have a family member with DS will aklmost always tell you what a wonderful and awesome addition they are to the family. Surveys and research bears out these results, yet the medical community continues to pound the drum of doom.