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Something about DNA I remember.
|dw:1348475197054:dw| - As seen in this drawing, the homologous chromosomes have independent assortment. Hence if there are different alleles on the homologous chromosomes, there will be different alleles going into different gametes. - There is also crossing over (not shown), causing homologous chromosomes to exchange parts of their DNA (and hence maybe different alleles of the same gene) with one another, causing more variations. - Fertilization is random. Hence, there is about a 50% chance in this case to have a certain allele adding to the same egg. Multiply that by another 50% in the egg, you have 25% of getting a certain combination of one allele.
Sex cells are produce from meiosis, these cells only have 23 chromosomes instead of the full 46. Since sex cells are only used in reproduction, and sexual reproduction requires two organisms, an organism must start with two sex cells. These two cells come from two organisms with different DNA. The DNA from these two cells is "molded" together. The new organism has DNA from both of the parents therefore it's not exactly identical to either parent.
Meiosis results in chromosomal recombination, basically crossing over of the chromosomes will occur. This creates varieties. Since nature is undergoing transformation every second, recombination result for better survival of the organism