system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does inbreeding cause birth defects?
gpt
Imagine your traits in terms of DnD stats. For simplicity let's give you five traits (in reality you have a ton): STR, CON, DEX, INT, WIS.

For each stat, you got one number from your mom, and one from your dad. The better of the two becomes your stat. So for example, your stat table may look like this:


Stat | from dad | from mom | your final stat
-----|------------|-------------|----------------
STR | **15** | 7 | 15 
CON | 7 | **9** | 9 
DEX | 0.5 | **9** | 9 
INT | **17** | 12 | 17 
WIS | **9** | 8 | 9 

You take the best of what you got from your parents, so you end up with final values of 15 STR, 9 CON, 9 DEX, 17 INT, and 9 WIS. Note that your dad gave you an absolutely atrocious DEX stat: 0.5! But that's OK, because it gets covered up by the 9 from your mom.

Until you have kids, that is. When you have a kid, then for each stat, one of your two numbers gets selected. You pass that number to your kid. So for INT, you could pass down your dad's 17, or your mom's 12. It's 50/50.

Without inbreeding, everything keeps going normally. But, what happens if you have a kid with your sister? Well, now's where that terrible 0.5 DEX from your dad could come into play. Your sister might have that 0.5 DEX gene also. You have a 50/50 shot of passing down that 0.5. With any other woman, it would just get covered by whatever DEX stat she passed down. But with your sister, she also has a 50/50 shot of passing down your dad's 0.5 DEX. And in the 25% chance that you and your sister both pass it down, then there's nothing to cover it up. Your kid will have 0.5 DEX and be a cripple.