Why does inbreeding cause birth defects?
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When inbreeding there are much higher chances for rare recessive traits to combine in the offspring.
As you might know, if you have one copy you’re typically ok, but if you have two copies then it will express whatever undesirable trait it codes for.
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People have two copies of every gene. Some copies are broken, but the second copy makes up for it, so they’re healthy and develop fine. If a developing fetus has two broken copies, sometimes this causes birth defects.
You get one copy of each gene from each of your parents. If your parents are closely related, the chances they both gave you bad copy of the SAME gene is much, much higher than if they’re not related, because they share so many genes to begin with.
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.
Inbreeding is bad in humans because, in the past, we have had VERY small bottlenecks in our population. I believe that, tens of thousands of years ago, we believe that at one point there were as few as 300 humans on the planet. This means that humans have a very low level of genetic diversity, compared to animals that haven’t experienced a population bottleneck. This lack of genetic diversity makes inbreeding dangerous as malfunctioning, recessive genes can get paired up, result in offspring with diseases.
It is not uncommon to be the bearer of several recessive genetic diseases. Our genes are quite often wrong, this is why we have two copies of most genes. If one does not work then at least the other one does. However these faulty genes with the recessive genetic disease will get transfered to half your offspring. And if these offspring start having their own babies it is likely that the baby will inherit the same faulty copy of a gene from both their parents.
The reason it causes birth defects is that this is the first time these faulty genes will show themselves. There are genetic diseases which only become evident later on but many of them can be diagnosed right at birth, or even before.
The bad traits are amplified.
Think of someone receiving a 10% chance of getting a gene that causes cancer. Now if two parents each have a 10% chance of spreading cancer, the cancer is more likely to be passed on to the child. Now the child has a 20% chance of getting cancer and contains the gene to pass it on. They have offspring with a person who has a 20% chance and the likelyhood goes up to 40%.
Those numbers actually don’t completely work in real life, it is vastly simplified for the CMV. It just comes down to likelyhood.
When DNA is replicated sometimes things change and can cause mutations which are passed down to the offspring. If you have two offspring with this trait and they mate this mutation is twice as likely to be passed on to their offspring. If that offspring breeds with other offspring that mutation is even more likely to be passed on.
If the first offspring with the mutation mated with a different family there is still a chance they pass it on to their offspring, but being with someone who has the same trait increases the likelihood of getting the trait.
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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