> neotinization of modern humans results in a condition known as Down's
> Syndrome ( with even further DNA differences).  Many modern humans carry
the
> gene to produce such offspring under the right conditions. Perhaps one can
> think of modern humans as Down's Syndrome Neanderthals

No, people don't carry any gene for DS. Errors in cell division happen all
the time, not just with chromosome 21, it's just that the others are
leathel. Here is the explination of how DS occurs:
Many errors can occur during cell division. In meiosis, the pairs of
chromosomes are supposed to split up and go to different spots in the
dividing cell; this event is called "disjunction." However, occasionally one
pair doesn't divide, and the whole pair goes to one spot. This means that in
the resulting cells, one will have 24 chromosomes and the other will have 22
chromosomes. This accident is called "nondisjunction." If a sperm or egg
with an abnormal number of chromosomes merges with a normal mate, the
resulting fertilized egg will have an abnormal number of chromosomes. In
Down syndrome, 95% of all cases are caused by this event: one cell has two
21st chromosomes instead of one, so the resulting fertilized egg has three
21st chromosomes. Hence the scientific name, trisomy 21. Recent research has
shown that in these cases, approximately 90% of the abnormal cells are the
eggs. The cause of the nondisjunction error isn't known, but there is
definitely connection with maternal age. Research is currently aimed at
trying to determine the cause and timing of the nondisjunction event. Here's
the karyotype of a
male with trisomy 21:


  Three to four percent of all cases of trisomy 21 are due to Robertsonian
Translocation. In this case, two breaks occur in separate chromosomes,
usually the 14th and 21st chromosomes. There is rearrangement of the genetic
material so that some of the 14th chromosome is replaced by extra 21st
chromosome. So while the number of chromosomes remain normal, there is a
triplication of the 21st chromosome material. Some of these children may
only have triplication of part of the 21st chromosome instead of the whole
chromosome, which is called a partial trisomy 21. Translocations resulting
in trisomy 21 may be inherited, so it's important to check the chromosomes
of the parents in these cases to see if either may be a "carrier."

The remainder of cases of trisomy 21 are due to mosaicism. These people have
a mixture of cell lines, some of which have a normal set of chromosomes and
others which have trisomy 21. In cellular mosaicism, the mixture is seen in
different cells of the same type. In tissue mosaicism, one set of cells,
such as all blood cells, may have normal chromosomes, and another type, such
as all skin cells, may have trisomy 21.