Why does inbreeding cause birth defects?

Question

Why does inbreeding cause birth defects?

In: 3

Answer 1

Imagine I have a rare defective copy of a gene.

If I have kids with some rando, half of my kids will inherit one defective copy from me – but crucially they’ll inherit a working copy from their mother and will be fine.

But what if I have kids with my cousin, who also has a defective copy?

Now you have a perilous situation: 25% of the kids will inherit *two* defective copies of the gene and *zero* working copies.

Repeated generations of inbreeding make it more likely that these recessive bad copies of genes get carried along – but it only happens if they exist in the first place.