system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why incest is bad

Ok, I'm not into incest, I'm just curious. What makes incest bad? I know that inbreeding can cause birth defects, mutation, etc... but why?

If birth control is used, what's wrong with incest? (Other than being really gross.)
gpt
Picture this:

You and your sister have 4 lego blocks each, their colors are red-green-blue-yellow. 

Mary, a friend you like, has 4 lego blocks too, their colors are purple-orange-pink-teal.

Suzy, another friend, has 4 blocks, but they are **red**-brown-purple-teal. So Suzy has a block that is the same color as one of yours.

You like diversity, so 2 red blocks stacked together along with a blue one and a green one are not your cup of tea.

You want to create a new lego stack of 4 blocks, and for this, you can use two of yours, and two of someone else. But you can't choose which blocks you use, so you just have to blindly grab some.

If you and Mary both put 2 blocks at random on the new stack, you'll have 4 differently colored blocks, and you both are happy.

If you and your sister both put 2 blocks at random on the stack, chances are very high that 2 blocks of the same color go on there. It doesn't always happen, because you could have grabbed red and green, and your sister blue and yellow, but since you both can't choose which ones you take, you *could* end up with same color blocks.

Now, Suzy has a red block too. If you and she both take 2 blocks, the chance of both grabbing the same color is smaller than it was with your sister, but it still exists. This is why there are still genetic defects, even with people who aren't related.