Question

Why is crossing over an important source of genetic variation?

Answer 1

You get new genetic combinations

Explanation:

What happens is that before mitosis proper begins, parent one's chromosomes and parent two's chromosomes are together. Sister chromatids will then start swapping bits. The swapping isn't always the same. One chromatid could have 1/4 of another parents, 1/2 of another parents, 1/28 of another parents. The only thing that is even is what is swapped onto the other sister chromatid. So if 1 has 1/4 of 2's, 2's has 1/4 of 1. Just image you've swapped a piece of chocolate bar with a friend. You both still have a whole bar, just one peice or two are a different flavor.

So no one is going to get maybe one of parent 1's chromatids, one of parent 2's chromatids. They're getting pretty much all new combinations. This allows for new genetic combos we couldn't have seen from one parent alone. Maybe parent 1 had Gg, maybe parent 2 has Gg and the result is a kid with one little g from mum and another little g from dad, gg.

This video explains the mechanics of crossing over, the actual swapping of the chromatids. With funk music.
This video explains the mechanics of crossing over, the actual swapping of the chromatids. With funk music.