Why does inbreeding result in a higher chance of genetic defects?
In: Biology
To put it really simply,our genetic codes all have some little defects, but they are usually what is called recessive traits, which means that you would need to have that defective code twice for it to appear and be active.
Now you and all of your blood relatives share a common genetic pool, which means that your relatives are very likely to also have the same genetic defects.
So with inbreeding there is a higher chance that the child receives in his genetic code two similar recessive defects (one from the father and one from the mother), and thus might have malformations or other things.
Genetic birth defects are usually recessive traits, meaning it requires two copies of a gene to activate.
If your father carries the genes for Tay-Sachs (a genetic illness responsible for killing children painfully), but your mother doesn’t, then both you and your sister are going to be healthy.
But you each have a 50% chance of carrying the gene. Let’s say you both do. If you two have incest-babies, you will have a 50% chance of your child being a carrier & a 25% chance of your child being a Tay-Sachs baby.
But if you marry outside the family, the chances of getting together with another Tay-Sachs carrier are small, and thus your child is less likely to have the condition.
It’s not just known genetic conditions, it’s also random junk changes in the DNA caused by replication that don’t cause any problems when they aren’t duplicated in the DNA.
And as for less damaging genetic traits, like imperfections in the eyes or nose, or extra fingers or other weirdness, the inbreeding essentially selects for those traits as “desirable” and makes them more prominent.
If the parents are not related they might carry some recessive genes that could result in genetic diseases but it’s rather unlikely that both have the same so the offspring has only a minute chance of getting 2 ‘defective’ alleles of the one gene. When the parents are closely related on the other hand it is rather likely that both carry the same recessive genes which means there is a pretty high likelihood of the offspring getting 2 defective alleles that will express the genetic disease.
Think about it that way:
Your genetic code has 2 letters, A and B. Having B in your code means you can have the defect.
You are AB (which means you have a 50% chance of having the defect), and so is your sister.
If you have a child, he/she will have a 25% of definitely having the defect (BB), 50% chance of having it half of the time (AB), and 25% of definitely being healthy.
Now think there’re the pairs AB and CD, that have the same mechanic (AB is ABCC and CD is AACD). If a ABCC has a child with a AACD, the chances of having the defect are lower (AB with AA results in a 50% chance of maybe; same for CC with CD) than if you crossed ABCC with ABCC.
This is very *very* oversimplified, but it gives the idea, I think.
I’m no expert, but my understanding is that it increases the chances of defect genes expressing. If both the parents have it, it greatly increases the chance of it expressing
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