Re: Why are two sex chromosomes necessary for normal human development?

Dear A., 
People with only 1 sex chromosome have abnormalities - Turner's syndrome 
women have all sorts of problems.  I think they are also infertile.  You 
have to think of this from a different perspective.  Now, if you only had 1 
sex chromosome then 
1)  Mating would be unnecessary.  We'd all just have to give birth to 
clones so males would be unnecessary.  There'd be no variation in the 
population. 
2)  Variation in offspring means that some with have advantages over the 
others in the face of environmental changes.  

3)  The 2 chromosomes are there so that a process called RECOMBINATION can 
take place during the formation of eggs or sperm.  Parts of the two 
chromosomes are swapped around so that all the eggs or sperm are different, 
leading to greater variability in the offspring - this facilitates no 2.

In women the x chromosomes are randomly inactovated in cells so although 
only 1 is expressed in any given cell, both are being expressed throughout the 
body. 

Hope this helps, 
Yvonne

key words to look up
Recombination - meiotic 
Genetic Variation 
Turner's syndrome
Kleinfelter's 
Barr bodies

Admin note:
Some of the bad consequences in Turner syndrome might be due to the fact that 
not all genes on the "inactive" X are actually inactivated. In fact, there are 
about 35 genes that still remain active. Females might need two copies of the 
active genes in order to develop into "proper" women, while males might only 
need one copy to be male. 
Look up "X-inactivation" and "escapers" on Pubmed or on the WWW.