Well, suppose bloodlines like Sharingan have autosomal dominant inheritance. Meaning a set of Bloodline/Normal parents would confer a 50/50 shot to each of their offspring of possessing the bloodline. Meaning you can have a pair of siblings from the same parents... but only one of them has the bloodline. (Aa/aa... 50%: Aa, 50%: aa)<br/><br/>Now, if a set of Bloodline/Bloodline parents have children... the rate increases to 75% for each child to possess the bloodline. (Aa/Aa... 50% AA, 25% Aa, 25% aa)<br/><br/>Now, if a set of Bloodline/Bloodline parents born of the<strong>above</strong>couple then went on to have children... the chance could be 100% for every child. (AA/Aa.... 50% Aa, 50% AA)<br/><br/>Given that at the age of 12 you become more or less a soldier, and being that twelve year olds (Tsunade's Bro) have been sent<strong>into</strong>combat... we can infer the lifespan of Ninja to be very very short. Name a Naruto character who died a natural death. :p Heck, just look at how huge each generation is compared to the last one. (Could be a focus of the narrative, but I think it's safe to say that veterans of the older Shinobi Wars are few and far between)<br/><br/>So... If you were a bloodline-using clan, and your members could be called away on a mission and die at any moment- ...would you accept 50/50 odds per child?<br/><br/>Whew... got a little carried away there. :P