This started out as a response to Daiyamondo's thread, but it got so heavy I decided to make a new thread out of it. The question is, how is a clan's bloodline limit maintained, what keeps it from weakening over time, and why arent clan members all deformed brother-daddy-mama-uncle-inbreeders? (Why do I care? Why does this matter? Why am I writing this? Who knows? *sigh*) GENETICS: Now, one thing that makes a huge difference is how exactly the bloodline limit is transmitted. It has been said in the series that it is genetic. But that doesn't tell us everything. In genetics there are dominant genes, recessive genes, and traits with varying expression depending on multiple genes. If you don't understand what I'm talking about, read this: RECESSIVE If a bloodline limit was defined by recessive genes, then for a person to have that bloodline limit, they would have to be born from parents that both gave the bloodline limit gene. In a clan like Uchiha, where every member of the clan has the potential to unlock the Sharingan, that would mean that every member has both halves of the recessive gene. If they ever mated outside the clan, that child would not be capable of getting the Sharingan, however that child could mate with a someone with full Uchiha blood and have a 50% chance of producing a child that could get the Sharingan and technically be 100% Uchiha. That person could also mate with another half Uchiha and have a 25% chance of producing a 100% Uchiha child. (Again, if you don't understand why that's true, read that website about genetics.) With whatever combinations you try, if a person is indeed able to activate the Sharingan, then they are 100% Uchiha. However, this also means that when clan members mate outside the clan that those offspring are either not allowed to have the name Uchiha or not allowed to have the name unless they are able to use Sharingan at some point. This must be the case since all official Uchiha clan members have the potential to activate Sharingan and therefore cannot be the direct offspring of a clan/non-clan mating. For other clans, however, we dont know the rules. If other clans with bloodline limits defined by recessive genes were to allow those that did not inheret the bloodline limit to be official clan members, then there would be clan members without the bloodline limit mating within the clan. The percentage of clan members that could activate the bloodline limit would decrease over time. DOMINANT If a bloodline limit was defined by dominant genes, then for a person to have that bloodline limit, only one of their parents would have to have the bloodline limit. And in that case (where the other parent has no dominant genes for the bloodline limit), depending on the parent with the bloodline limit, there would be either a 50%, or 100% chance of producing a child with the bloodline limit. If the parent had both halves of the gene dominant for the bloodline limit, then all that persons children would have the bloodline limit no matter what. But if the other parent only had one dominant gene, then the child could end up with either two or only one dominant gene. If they only got one and then mated with someone with only one, then there would be a 75% chance of having a child with the bloodline limit. If they mated with someone with no dominant gene, meaning they didnt have the bloodline limit at all, there would be a 50% chance of having a child with the bloodline limit. In all cases, the only way to guarantee that the child will have the bloodline limit is if one of the parents has two dominant halves of the gene. So in a clan like Uchiha, this would mean that all members of the clan have two dominant halves of the gene, because otherwise certain combinations in the clan would produce children that did not have the bloodline limit even though both their parents had it. COMBINATION/VARIABLE Some traits are defined by a single gene. (People can either roll their tongue or not. People have either attached earlobes or not. People either have dimples or not.) But many traits have varying forms of expression. Many of the characteristics of the human body are defined this way. Eye color can be many different colors and many different shades of each color. This is controlled by several genes. So, if a bloodline limit was defined this way, then everything becomes much more complicated. Different combinations of parents within the clan can create many, many different variations. Mating with someone outside the clan could cause one of the genes normally impacting the bloodline limit to not be passed on. But this could cause such a small difference in bloodline limit strength that it goes completely unnoticed. However, a continued line of mating with people without the bloodline limit will cause it to become weaker and weaker over time as it is diluted, eventually causing the bloodline limit to disappear. To keep this from happening, the clan would definitely have to keep mating within the clan. Sure, some exceptions could be made without much impact, but it would have to be kept to a minimum. Also, even though all the clan members would have the bloodline limit, some would have it much more strongly than others, like in the Hyuuga clan. This could also be the reason why all Uchiha members have the potential to activate the Sharingan but it come easier to some than others. Depending on how strongly the bloodline limit was inherited, it may take much more work to activate it. THE RULE In call cases, the best way to keep the bloodline limit as strong as possible is to keep marriages within the clan. But doesn't that mean they are all a bunch of in breeders who are gonna have "extra retard babies"? Nope..... For deformations and other inbreeding issues to take place, the population needs to be somewhat small. Deformations are recessive traits that are very rare. They are recessive, meaning both halves of the gene must be the recessive trait for it to be shown, but because it is so rare, that almost never happens. However, when a small population breeds within itself, the same recessive gene gets passed around and around to that the number of people in that population that has it becomes higher and higher. It then becomes more likely that two people with the unexpressed recessive gene will produce a child that has both halves of that recessive gene and will have the deformation, whatever it is. But these people are not inbreeding. The clans are very large. Yes, we've only seen the Uchiha village, which we can all agree was huge, but I'm sure the other clans are similar in that respect. They all have the same family name, but with so many people, they are all very far from each other on the family tree, which would likely have to be some giant redwood tree to fit the complicated family structure on it. Yes, the percentage of people with particular recessive genes for deformation does go up within the population, so the chance of someone being deformed does go up, but it is a very small amount. You will not end up with an entire population thats deformed. The deformation would most likely be mutated out long before it spread to the entire population. FORMATION Sure, this all works since the villages are so huge, but its not like the Uchiha clan just appeared out of nowhere. So how did the clans get so huge when they all have the same bloodline? Well that depends on what bloodline formation theory you believe in. I kinda side with the theory that goes something like a creature/demon/whatever with particular abilities being the source of the power which is somehow merged/sealed/mated/whatever with a human which gets the powers. Through marriages and mating (with people without the bloodline limit), the genes that define the bloodline limit are spread to a larger gene pool. But depending on if the bloodline limit is defined by dominant, recessive, or a combination of genes, not everybody in this family line actually has the bloodline limit (or it would become weaker and weaker if it was defined by combination). At some point, the people who have the bloodline limit can decide to get together and form a clan. From that point on, if they kept marriages mostly within the clan, the bloodline limit would keep its strength. The only issue is that if the bloodline limit had been defined by dominant genes, then some of the children of the initial marriages would not have the bloodline limit. To form a clan from this where all members and all their children had the bloodline limit, they would have to kick out all members who were born without the bloodline limit and forbid members who had children without it from having more children (this is not necessary, but it would make this process faster) and eventually, over centuries, the entire clan and all children born within it would have the bloodline limit. And they lived happily ever after. THE END!!