Re: Since the Egg 'lets' in a sperm, isn't sex determined by the female?

Mark:

The determination of genetic sex of the offspring is the "responsibility" 
of the heterogametic sex. In the case of humans this is the male. The 
sperm carries either a Y or X chromosome.  Upon fertilization and union of 
the pro-nuclei, the diploid number is restored and if a pair of X 
chromosomes reside in the new embryonic nucleus, the individual is 
genetically female. If there is one X and one Y chromosome, the embryo is 
genetically male.

You ask about the role of the oocyte in "letting in" the sperm.  This is a 
bit of a misunderstanding on your part.  There are binding proteins in the 
area around the oocyte called Zona Pellucida Proteins. These proteins 
actually will bind with surface antigens on the sperm following 
capacitation of the sperm.  This binding happens to the first sperm to reach 
the egg, and there is no evidence to suggest that the X- or Y carrying status 
of the sperm effects its ability to bind. Once the sperm ZP protein complex is 
bound to the oocyte membrane there can be no other sperm binding as the block 
to polyspermy is initiated.  Once the block to polyspermy begins, there will 
be a physical barrier to sperm as the zona pelucida actually hardens into 
the fertiliztion shell.  Furthermore the zp elevates away from the oocyte 
membrane to generate a space restriction as well.  

As the sperm is entering the oocyte, there will be a resumption of meiosis 
and the second meiotic arrest will be overcome.  The oocyte (secondary 
oocyte) will rapidly go through Metaphase II, Anaphase II and Telophase 
II. There will be an expulsion of the 2nd polar body and the ovum nucleus 
will now be haploid.

When the sperm is drawn into the oocyte cytoplasm, the sperm nuclear 
material will spill out through pores in the nuclear envelope.  This "pro-
nucleus" will co-mingle with the pro-nucleus of the oocyte and generate a 
new individual.


Please also see this URL:  http://www.gfmer.ch/Endo/Lectures_10/Fertillization.htm