Down syndrome, discovered in the year of 1862, by John Langdon down, it was known as "Mongoloid child", due to the similarity with mongolian race, and associated with mental disability.
It is caused by having 3 chromosomes in the group number 21 in the cell, instead of having 2, so the total will be 47 rather than 46 in the cell.
It can occur for one child per 800, or per 1000, related to country and the environment.
Till now they didn't discover the main reason for this abnormal division, but there are many factors which can increase the probability, like the mother's age, or in general, the parent's age, and it can be discovered during the pregnancy period, by using screening test, or diagnostic test.
It is really important to know how to deal with those kids who have down syndrome, in the past, at the beginning when they discovered this disorder, they used to isolate people who have it, and let them live in a separated world, but this was violating human rights, then they started to let them share in the normal society.
Kids with down syndrome have the same feelings as everybody has, and they can form any type of relationship as we can, not only that but also one study registered that one women with DS became a mother, and two men became fathers, so they can deliver.