Why does inbreeding cause birth defects?
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Inbreeding doesn’t “cause” birth defects. It increases the likelihood of genetic abnormalities, aka birth defects.
It is possible that the offspring of two closely-related parents will be completely healthy and have no birth defects, depending on the genetics of each parent. So inbreeding in and of itself doesn’t “cause” anything. Every offspring won’t necessarily inherit whatever is the problem unless both parents have no other DNA to give them.
However if the two closely-related parents both have the same genetic flaw, depending on the genetics the chance that their offspring will also have it increases dramatically.
Also, if the offspring inherits the flaw from both parents, not just one, there is an increased chance that the birth defect will be worse in the offspring than it was in the parents. Depending on what it is.
If both parents have the trait and have no other DNA to give, then all of their children will also have the trait. And will also have no other DNA to give their children in the next generation.
Families with generations of inbreeding may show the same abnormality from generation to generation, with it becoming worse in each generation. They aren’t just passing it on to their kids, they are doubling down on it with each generation. There are examples of this is certain closed communities with small populations. And families where incest and/or marriages of related persons are behaviors/customs that are handed down from one generation to the next. Whatever the trait is, it becomes increasingly common, generation after generation.
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