Originally posted by kristopin
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Within seconds after conception (the moment a sperm penetrates the egg sac) the chromosomes carried by the egg and the sperm are melded and the single cell remaining begins to divide to form the new being. Now, this single cell has received one copy of each chromosome as carried by the egg and one carried by the sperm, the male contributing either an X or Y, the female always an X. Sex is determined at this time.
The sex organs do not develop for several months and the androgen (testosterone) causing the male features to develop are caused entirely from within because of the activity intitiated by that "Y" chromosome. It does not come from the mother.
Here's a good animation showing how sexual differentiation takes place as the embryo develops.
http://health.howstuffworks.com/adam-200124.htm
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