Answer 1:
In humans, sex determination actually happens a
kind of late in embryonic development - in fact,
all early embryos begin making the sex organs
for both sexes and then in response to hormones,
one set will regress while one is retained. The
hormone levels are related to whether there is the
presence of a Y chromosome (male). Current
research is focused on what genes on the Y
chromosome actually direct the expression of
specific genes that then drive the hormonal
levels. In other animals, especially a lot of
reptiles, other, environmental factors can drive
sex determination - a particular temperature of
egg incubation might favor one sex over the other.
Or, if the general population has relatively few
females, there is a tendency for new clutches of
eggs to be largely female. Although we know about
the hormones, we don't understand the triggers of
this very well. So keep asking this question
- maybe you will be the one to figure out in the
near future! | 
