Down syndrome is caused due to an abnormal genetic profile but it is not a condition that is hereditary. A child does not inherit Down syndrome from the mother or father.
Down syndrome is caused when a person has three chromosomes 21 instead of two. Usually, people get one copy of each chromosome from their parents and have a pair of each. When a person gets an extra copy of chromosome 21, it results in Down syndrome.
The causes behind this happening are not very clear. The age of the woman when the child is born has been found to be one reason. As the age of the mother increases there is a higher chance that the child she has will have the extra chromosome resulting in a total of 47 chromosomes. At age 30, there is a 1 in 900 chance that the child will have Down syndrome. This increases to 1 in 350 by the time the woman in 35 and when the woman is 40, the risk is 1 in 100.
The chances of them having a child are nearly 0. There are only 1 or 2 known cases of Men with Down syndrome fathering a child, they are known to be sterile. The percentage of a female with DS having a child with DS are high, I don't remeber the percentage now; but they can also have a baby without.
Mom to 4 year old with Down syndrome