Sir: The issue here is that a mutation brings in new genetic information. A mutation within a gene will lead to a completely new protein being expressed.

With down syndrome, the extra chromosome doesn't have any new genetic information -- as far as I know... it's just an extra copy of the same information that then impedes gene expression.

It's an interesting topic, though. In French it's usually called Trisomie-21, because it appears on the 21st site. There are also things like Klinefelter's syndrome where there are three copies of the sex-chromosomes -- so chromosomal aberrations are rare but relatively common in large populations.

As far as I know, there are no advantageous extra-chromosome syndromes, unless you really torture the definition of 'advantageous'. (Castor Semenya, for example, wins gold medals in women's running, despite having the testosterone of a boy; and downies almost always seem happy).

Aerotrain