Hey guys! I've been making a few calculations on genetic risks. We all know by now that first cousin marriages pose little risk right? What about Neji and Hinata, who are genetically half-siblings?
Quick explanation on words:
- chromosomes: a person's genetic information is spread across 23 (I think) pairs of chromosomes. Each person inherits half of the chromosomes of each parent, one from each pair of chromosomes.
- DNA: it's what chromosomes of made of, they're basically a type of molecules.
- genes: a sequence of DNA, located on a chromosomes, that codes for a specific trait of yours, such as eye color and the rest. Bad genes can cause genetic diseases.
- dominant genes: if you have such a gene on one of the chromosomes out of a pair, then it's enough for it to express itself.
- recessive genes: you have to have the gene on both chromosomes out of a pair for it to express itself. This means you have to get it from both parents. Most genetic diseases are recessive: you can only get them if both your parents had the gene and passed it along to you. I'll thus be talking about dominant and recessive genetic defects from now on.
Inbreeding does Not cause genetic problems to occur on their own, they only increase the likelyhood that recessive traits show up. So we'll calculate the risk.
Considering any given pair of chromosome, I'll be calling one with a genetic defect B (bad), and one without G (good). When we're talking about recessive traits, you're healthy if you have the GG, BG or GB combination, and unhealthy if you have the BB combination.
Now! Before we begin, I'm sure you you'll all agree that Hizashi, Hiashi, Neji and Hinata look relatively healthy. It is thus safe to assume that they don't have any significant dominant genetic defects. So we can only assume that they have recessive ones. Now assuming that Neji and Hinata both have recessive defects, they could have them from either their mother or their father. Since both are healthy, they can't have inherited from both. So either each inherited the exact same recessive defect from their mother (unlikely) or from their father (more likely). So I'll assume that their mothers are fine, and that Hiashi and Hizashi on the contrary are the ones who have a problem (the risks can only be lower if they get it from their mother anyway, since the likelihood that both mothers have the same recessive trait is lower than with Hiashi and Hizashi, as twins have the same genetic material so would share the same genetic problems).
So let's assume that Hiashi and Hizashi both have a recessive genetic defect (so the corresponding chromosome would be GB).
The likelihood that Hiashi passed it on to Hinata is 1/2. Same with Neji. So the likelihood that Both Hinata and Neji have the recessive trait is 1/4.
In the event that both Neji and Hinata share the same recessive genetic problem (they'd also be BG), their children can have the following combinations (first letter from mother, second from father): BB, BG, GB and GG. Since it's recessive, only in the case of BB is the child unhealthy: a 1/4 risk.
So the overall risk is 1/4 x 1/4 = 1/16: one in 16, or 6,25%. In 93,75% of cases, their kid would be Fine.
What if there are Several seperate genetics defects? For each, the probability of the kid not getting it is 15 out of 16, so in the case of n problems, the probability that the child has none of them is (15/16)^n. So if there are 2 defects, they have a 87,98% chance of being completely healthy. But really: the Hyuga all seem to be more or less inbred anyway, so any genetic problem should have showed up in at least some of the members long ago. Yet they all seem rather healthy, and the Hyuga clan would not be considered one of Konoha's strongest if there were many Hyugas sick from having genetic defects. And if they don't have recessive defects in the first place, there's little risk of any showing up.
What's more, studies have shown that the increase in genetic risks from first cousins marrying is lower than what is expected out of calculations (see links in my other posts on that), and explanation could be that a forming foetus would automatically abort in some cases if there's a significant genetic problem. In any case, this should also be applicable to Neji and Hinata.
So the risks that their kids would have genetic problems would be Lower than 6.25% per kid, assuming that there are genetic problems running in the clan in the first place. In at least 93.75% of cases, their kids would be Fine. So tell that next time that someone claims that if Neji and Hinata have kids, the kids would have two heads and three arms.
