"Jan Philips"  wrote in message
>
> If so, how do additional chromosomes come into existence?  What I'm
> getting it is that either you have the 23rd chromosome or you don't.
> Along the line, there must have been a first being that had 23.  Who
> did he mate with?  Did others born later in his generation have a
> matching 23rd chromosome?  If he mated with someone with 22
> chromosomes, then how could he pass down the 23rd chromosome to his
> offspring, since his mate wouldn't be able to supply the other half of
> the 23rd chromosome?
>
It might have come about as a result of "hopeful monsters". Downs patients
have an extra chromosome, for example. As it happens they are handicapped
and unlikely to have children, but it is possible to imagine a situation in
which a similar syndrome was an advantage.
It is also not unknown for people to have extra copies of the sex
chromosomes. There tends to be a slightly deleterious impact on
intelligence, but nothing like as pronounced as with Downs. If a XXY
mutation spread, the extra X could in time become an extra independent
chromosome. (I don't know of an actual example of this happening in nature).
In plants it is also quite common for the whole genome to be duplicated.
Wheat, for example, is actually hexaploid - it is formed from a hybrid of
three ancestors.
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