How does inbreeding in humans cause birth defects?
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Those would be genetic defects, not birth defects. A birth defect is when something is introduced to the mother than causes the fetus to develop incorrectly. A genetic defect is when the genes in your DNA prevent your body from working or forming correctly.
Your body 2 copies of every chromosome, and as a result 2 copies of each gene (except on the sex chromosome, but we won’t get into that unless you ask). You get one set of those chromosomes from your mother, and another set from your father.
Most of the time, having one damaged copy of a gene isn’t a problem, but having 2 damaged copies is. For example, the gene for hemophilia (a genetic disorder where your blood doesn’t clot properly) if you have a hemophilia gene and a non hemophilia gene, you won’t have hemophilia, but you’ll be a carrier for it (you could pass it down to your children). This is because hemophilia is a recessive gene, just like blue eyes. However, blue eyes aren’t harmful to us, so blue eyes are a lot more common than hemophilia.
Sometimes only having one copy is harmful. Dwarfism in humans is a dominant gene, meaning you only need one copy of it to be a little person. So you could inherit it from only one parent. To continue the comparison from earlier, a dominant gene is like brown eyes. However dwarfism isn’t as common as brown eyes because having 2 copies of the dwarfism gene is actually fatal, so it doesn’t get passed on as frequently.
Now to get into inbreeding. If your parents aren’t related, then you are getting a random set of genes from both parents, it’s unlikely you would get the exact same set twice. However if your parents are siblings, then all of the DNA you would be getting would be coming from your grandparents directly. Ignoring mutations, you would have a 25% chance to get the exact same copy of a chromosome twice, repeat that for all 23 chromosome pairs, and that’s a lot of repeated DNA. That repeated chromosome would include all the same damaged genes twice. If you repeat this process over and over again, there end up being very few different copies of each gene, so if there is a damaged one in there, your kids will almost certainly have it.
It’s equally likely that undamaged genes get passed on to each generation, but almost everyone has damaged genes, and would be capable of passing those on. Your family has those same damaged genes, so a few generations down the line, without mixing in new copies of those damaged genes, the damaged genes start to express themselves.
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