The Kenketsu clan broke from the Uchiha clan generations ago. Due to being cursed they were cast out, lost in the records of the shinobi. Being cursed with large amounds of iron and blood build up, iy was easy for them to get sick, not all of the Kenketsu clan had this curse. Some called "the cured." Other say the unlucky. If you are able to muster the courage to try and control your blood to make weapons, if you fail at that task while trying to control your blood and body, youll either die or get seriously injured. Only 0.3% of people in the clan have the ability to use the Kekkei Genkai, most of them only have a miniscule amount of iron for a Kenketsu clan member,  if it was arranged into a chart it'd go; 0-10(give or take) with the kekkei genkai, 60% with large amounts of iron, but no kekkei genkai, the rest would be like everyday shinobi. Saying that doesnt mean they cant have a child with more or less iron in their blood, the curse marks vary. Only one child was ever born with more chakra and iron then the rest of the clan, he ended up dying trying to make himself a weapon and keep his mortal form. The Kenketsu useally do not have any babies born with this Kekkei Genkai, and it useally happens every few decades. So the clan useally uses the Sharingan (Spinning Wheel Eye) and other jutsus, because the kekkei genkai is unique to a few people. Currently no-one knows if there is a child/adult who posesses this gift, but most likely not.