Meiosis Occurs..

[Atlantic]

I have a question about the occurence of meiosis.

I was told meiosis occurs during gametogenesis (formation of gametes such as sperm/ova). And during meiosis 1 - prophase 1 there is crossing over occuring to create more diversity. My question is crossing over of whose chromosomes because we have 2 pairs of every chomorome (not chromatid)??? Each individual has its own set of chromosomes, I can't see how crossing over can occur during the formation of 4 sperm or 1 egg because fertilization has not occured yet. And does meiosis occur during fertilization, if yes the result would be 4 genetically distinct cells in one human being??? If no, then how can there be a genetically distinct zygote in result that has traits from both father and mother. I am not sure if the chromosomes are just mixed during fertilization (when sperm fertilizes the egg).

 

I'm just confused, please clear that up for me. (ultimately i'm asking where meiosis occurs)

 

Thanks in advance.

[Ms. DNA]

I found an online tutorial that might help here:

 

http://www.biology.arizona.edu/cell_bio/tutorials/meiosis/main.html

 

Meiosis occurs before fertilization takes place. Without meiosis, you wouldn't have sperm or eggs. Crossing over occurs between the maternal and paternal chromosomes during the first part of meiosis; it increases the different types of gametes you can produce. For this exercise, let's assume the genes for hair color and eye color are on the same chromosome. (I think they are, but I'm not completely sure.) Let's also assume I inherited gene for blond hair and blue eyes from my mother and black hair and brown eyes from my father. Without crossing over, my children would inheirit either blue eyes/blond hair or brown eyes/black hair. But with crossing over, you have more possibilities: the original two, plus blue eyes/black hair and brown eyes/blonde hair. That's what they mean by increasing genetic diversity.

 

Hope this helps.

 

(Edited to provide further explanation.)

[Atlantic]

thanks a lot