Researchers know that genetics play a strong part in FIP - at least 50% of the incidence or more has a heritable component. They also know that susceptibility is carried both in paternal and maternal lines, but have suggested, at a minimum, that paternal lines that throw kittens that die from FIP not be used for breeding. This is because toms breed multiple queens and sire dozens or hundreds of kittens, and have the greatest influence on how bloodlines are developed. This is true of a lot of diseases - the "Founder effect." If females are genetically weak, and bred to weak toms, that is when you get into problems. If toms are genetically strong, and queens are genetically weak, the male's resistance genes seem to mask this weakness.

The best scenario is to not breed either susceptible toms or queens, but not using problem toms is the best possible alternative in the interest of doing the most with the least disruption of breeding practices and bloodlines. However, if there are multiple losses from FIP in a litter, remember that susceptibility comes both from the paternal and maternal lines. It's also important to consider the cat - if you believe the cat may be at risk for FIP, avoiding the stress of being a breeding cat may help prevent the disease for that individual.