Why is incest not cool?

Avoiding incest is a learned behavior:
Sure incest (in-breeding) is a fantasy for some but most of us loathe the idea of mating with family members. This is obviously a learned behavior and there is nothing inherently disgusting about having sex with your family members. That it is a learned behavior is obvious because there are certain human cultures where incest relationships are quite normal. We are not even talking about some remote tribes. For example, in most parts in India it is perfectly fine to marry ones cousins. In several isolated cultures like islands, incest is inevitable.

However, in general most cultures do not allow such relationships especially between closely related members, say brother/sister or dad/daughter. One would obviously wonder why cultures evolved this way. After all, bonding between family members is usually much stronger and unconditional compared to a stranger you meet in high school. So marriages among family members will probably be more successful which is good for the society.

How does in-breeding hurt?
Turns out, the reason could be the fact that babies born to related parents are more likely to develop genetic disorders. So one can hypothesize that people over several generations have observed these genetic disorders in babies and gradually veered away from inter-family mating. 

Now the science side of the story. Why is a baby born to a dad and his daughter more likely to develop genetic disorders? To answer this question we need to first understand how we inherit genes from our parents.

Genetic inheritance:
Organisms usually have two copies of genes for every characteristic (lets use the gene that determines obesity potential as an example) obtained one from father and one from mother. These genes can be of two types - dominant allele (usually the good trait) and recessive allele (the bad trait). The bad trait is expressed only if both copies are of the recessive type.
Lets consider the following combinations of dominant (G) and recessive (g) alleles for obesity potential in an individual - 
As you can see, organisms can carry these 'bad' genes without expressing the traits and thus evolution never gets a chance to weed them out completely from the gene pool.

Now imagine a pair of unrelated individuals that met in high school. Both of them will be carrying certain recessive alleles but it is less likely that they will be carrying the recessive alleles for the same trait because they are unrelated. For example, the guy might be carrying a recessive allele for obesity while the girl might be carrying a recessive allele for diabetes without expressing these traits because they also have the dominant allele in them. However, if you consider a dad and a daughter, there is a very high chance that both of them carry the recessive allele for the same trait, lets say obesity, because they are related. Now, their offspring is very likely to inherit the recessive allele from both parents and end up with a 'gg' configuration and thus obese. 

This illustration should help. The high school buddies carry recessive alleles but for different traits. So, while it possible that they pass on these recessive alleles to their baby, it is impossible for the baby to end up with two recessive alleles for the same trait. However, in the second case, the dad and daughter carry recessive allele for obesity and thus their baby has a good chance of being obese.

Obesity is obviously just an example but you can see how in-breeding can result in various genetic disorders by increasing the likelihood of inheriting two recessive alleles.

So the take away from this article? Well, don't sleep with your daughter.

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