Ask A Scientist Molecular Biology Archive

name Dion 
status other 
grade other 
location N/A 

Question - How is gender selected at conception? Is it random or are 
there reasons -- i.e. acidity levels temperature at conception? Does the 
sperm have a precursor to give off the X or Y chromosome? 
---------------------------------------
The sex of a child is determine at coneption by whether a sperm carrying a Y 
chromosome or a sperm carrying an X chromosome fertilizes the egg. If the 
fertilizing sperm carries an Y chromosome, the embryo develops into a male. 
In humans, 51.4% of the time the egg is fertilized by a Y carrying sperm 
(the sex ratio at birth is 106 males for every 100 females. The reason why 
the sex ratio is not exactly 50% is not known but possible theories are that 
Y sperm swim faster because the Y chromosome is much smaller than the X 
chromosome or that the Y sperm live longer than the X chromosome bearing 
sperm for some reason.

Ron Baker, Ph.D.
====================================================================
Well, probability wise, it is supposed to be random. Half the sperm have a 
Y and half have an X. But some research shows that the biochemistry of the 
female reproductive system may be more receptive to Y or X sperm. 

 vanhoeck
====================================================================