Reply #31: as a biologist, I disagree.... [View All]

[mike_c]

Trisomy 21 is not "just another way of doing things." It's a survivable genetic accident, and a pretty serious one at that. "Just another way of doing things" implies that there should at least be self-maintained polymorphism for trisomy 21 in the human population, but there is no evidence of this. Women with Down syndrome are sterile, and there are only two recorded instances of Down syndrome male reproduction. In other words, it is-- for all intents and purposes-- NOT a heritable condition.

Finally, nondisjunction happens to all of the other human chromosomes as well, at about the same rate as c21 nondisjunction. Some others are survivable, although all such instances result in profound genetic disorders, and most are NOT survivable. Nondisjunction of chromosomes is almost always fatal. Only trisomy of a few chromosomes is survivable, including c21. I'd hesitate to call nondisjunction simply "another way of doing things" since it is almost invariably fatal, at least in humans.

Edited to note that my comments in the paragraph above refer only to autosomal chromosomes, not sex chromosomes. Although nondisjunction of sex chromosomes often leads to genetic abnormality in humans as well, it doesn't always.