Why does inbreeding increase the likelihood of the offspring having a genetic defect?


Why does inbreeding increase the likelihood of the offspring having a genetic defect?

In: Biology

Every individual carries a double complement of genes, that is each gene comes in a pair, one from mother, the other from father. If one gene out of the pair happens to be defective, there is still the other healthy gene from the pair which can take the workload. But if mom and dad are closely related, they have a large chance of having the same defective genes (like inherited from the same parent), and therefore the offspring has a large chance to get BOTH genes in a pair defective, resulting in a physical deformity or other negative effect.

Because your genes are full of mistakes. The system has evolved to deal with this through redundancy. You have two copies of each^† gene, one from each parent, and most processes use one copy some of the time and the other copy the rest of the time. Only when both are broken do you have the most serious problem. When you inbreed, say by having a child with your sister, your child is a lot more likely to end up with two copies of one of your Mom’s genes. If that one has a bug, the child is in trouble.

† Except if you are male. You only get Y from your father, and X from your mother. There are genetic conditions, like some forms of hemophilia, which only effect males through their fathers.

Imagine that everyone has two hand-written copies of Harry Potter they carry around. Each one is written based on your parents’ four copies, taking pieces of each in different combinations.

As you can imagine there are a lot of little variations in the stories from all of the copying over and over. However since you remix your parents’ versions twice, you probably won’t be missing any huge details. If a passage is really wrong in just one of your parents’ books, you’re not likely to copy it in both of yours.

But what happens if a group of people stops bringing in fresh copies, and mostly uses the same books over and over (by ‘inbreeding’)? Differences are more likely to survive, and you’ll get some really weird passages that are actually in every book in your family. Then, when you make your remixes you’re less likely to have access to corrected bits.

This is how inbred populations can end up having their own genetic canon which, in the absence of outside corrections, will be more likely to result in defects.

Imagine your parents are just 2 groups of numbers in 2 columns. Column A is Mom, Column B is Dad. Moving from the top of the column, you compare the pair of numbers, and whichever one is higher, is now YOUR number. If one parent has a low number on one of the pairs, its okay, because the other parent more than likely has a higher number (or average number). But what if both sets of numbers are low? What if both sets were nearly identical because they never change?