Re: What happens if a person/plant/animal isn't born with 23 chromosomes?

Chromosomal loss in animals is usually lethal, and most of these 
individuals will not develop to the point of being born. There are a few 
exceptions to this of course depending on which chromosome is in fact 
missing. The loss of an autosome (non sex linked chromosome) is usually 
lethal as these chromosomes carry so many genes that the loss of 1 can 
led to the loss of biological functions, inability to produce certain 
proteins, lack of organ/body development and so on. There are cases in 
which it is possible to possess more then 1 copy of a chromosome, though 
this also can led to many problems, causing things like Down's Syndrome 
for example. 

Chromosome deletions can occur in relation to the sex chromosomes (the X 
and Y chromosomes), it is possible to only have 1 X chomosome. Instead of 
having the XX pair for females and the XY pair for males, you get a X0 
which results in a disorder called Turner's Syndrome. Plants are more 
forgiving to the possession of multiple copies or missing chromosome then 
animals but the resulting individual will usually end up sterile and 
unable to reproduce itself. 

Moderator's note:
Humans have 23 sets of chromosomes, but other organisms may normally have more
or less. Some have only a couple, while others have hundreds. However, the gain
or loss of chromosomes in all cases causes a change in the number or dosage of
the genes, and is therefore bad news.