What is crossing over? Of what importance is it in meiosis?

Question

What is crossing over? Of what importance is it in meiosis?

Answer 1

Crossing over is when homologous pairs of chromosomes exchange genetic information between them in Prophase I of meiosis.  The importance is that it creates vast numbers of possible combinations of alleles in the gametes.

For example, if on chromsome 1 that you got from your mom you carried the alleles ABCD and on chromsome 1 that you got from your dad you carried the alleles abcd, without crossing over, when these chromsomes segregated, half of your gametes would have ABCD and half would have abcd.  But, with crossing over, you can get all kinds of possibilities, including: AbCd, abcD, Abcd, ABcd, etc.

Now, imagine this happening with all your 20,000+ genes!