But at a genetics symposium, two independent groups presented data from the X chromosome hinting that modern humans interbred with other human species
I have always wondered if children born with Down's Syndrome are actually a throw back.
No. It's a structural defect caused by having two chromosome 21 in the egg -- the older the female the more often this happens.
When fertilized by sperm, this will produce a zygote with three (two from the abnormal egg + one from the sperm) 21st chromosomes. Thus the technical name "trisomy 21".
A related defected is "monosomy X" or Turner's Syndrome.
Normally, a male will have an X and a Y chromosome, females with two X'es. In Turner's Syndrome, a baby girl develops with only a single X chromosome.
Intellect is usually normal but growth is stunted, abnormal bone development in the chest and abnormal or no menstruation -- and usually the woman is infertile.
No---Down's Syndrome individuals have an extra chromosome, so instead of 46 like you and me, they have the odd number of 47. This produces the typical syndrome, but there is a broad range of its manifestations.
Throwback is a prescientific term, with not current meaning. There are many kinds of mutation, and chromosome copy errors are one. In plants, chromosome mutations are common and frequently lead to new species. This is rare, in animals, but it does happen.
All known human chromosome mutations are considered detrimental.
I don't think children with Down's syndrome could survive in the wild, even high functioning ones.
They don't have the survival instincts that animals have.