|MadSci Network: Genetics|
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
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