On 21-7-2009 18:15, clipka wrote:
> Darren New <dne### [at] sanrrcom> wrote:
>> Or it's less hard to combine 48 with 46 than you might think. All the data
>> is there, including the tollermerines (or however you spell it) in the middle.
> 
> Just happened to stumble across the Wikipedia articles on mules and hinnys
> (citing the latter here):
> 
> "A donkey has 62 chromosomes, whereas a horse has 64. Hinnies, being hybrids of
> those two species, have 63 chromosomes and are sterile."
> 
> But later on the article reveals that this is an oversimplification:
> 
> "Female mules have been known to produce offspring when mated to a purebred
> horse or donkey, though this is extremely uncommon."
> 
> 
> Duh. Didn't know that horses and donkeys have different numbers of chromosomes,
> too (then again, it was news to me as well that chimps and humans have).
> 
> So this makes me think that...
> 
> - mutations changing the number of chromosomes don't seem to be as uncommon as I
> had thought

http://en.wikipedia.org/wiki/List_of_number_of_chromosomes_of_various_organisms
is hard to explain otherwise.

> - with mules and hinnys being perfectly viable and typically not showing any
> other defects, and even *not always* being sterile, this gives rise to the
> assumption that having an odd number of chromosomes is an absolutely
> non-dramatic mutation as such (why should *only* the reproductive system show
> issues? and why not always?), and that the sterility is instead imposed by
> other genetic mechanisms "designed" (by evolution) to separate the two gene
> pools.

I guess how dramatic it is, is variable. For those that we know of it is 
not an extreme problem. Where 'know of' is the important bit. If the 
translocation that resulted in our chromosome 2 would not have been 
viable or have been sterile, we would not have known.
You know of trisomy 21 because it is common, viable, and recognizable. 
According to wikipedia trisomy 16 is actual the most common, guess why 
you have never heard of it.

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