The short version is that it's a gene that causes mutations and that in somatic and egg cells is turned off. A P-positive female can have offspring with a P-neg

The short version is that it's a gene that causes mutations and that in somatic and egg cells is turned off. A P-positive female can have offspring with a P-negative male (because the egg cell carries the repressor), but a P-positive male cannot have offspring with a P-negative female. It's easy to see why the P-element would spread through the population.