Board Thread:Character Discussion/@comment-28282242-20160423112213/@comment-27794543-20170506105233

What exactly is a genetic mutation if not a mutation of a gene? Perhaps what you meant is that it's not a single gene, but multiple genes interacting.

If it's a single gene (like JR has stated), it can be a recessive trait (require two copies of the gene for the trait to be expressed). Even if all Nightbloods are killed, there are numerous Grounders who are carriers (have one copy of the gene) and can pass the gene to their children. If two Grounders who're carriers have children, 25% will be Nightblood, 50% will be carriers, and 25% will be neither. The percent of the population who are carriers influences how often two carriers end up together – the lower percent of carriers, the lower chance of Nightblood birth.