"Excessive inbreeding" refers to a situation where animals or people with close blood relations mate too frequently with each other. This can lead to a higher chance of genetic disorders and health problems in the offspring. Full definition
«I am against the French,» she says, «because they are guilty of excessive inbreeding and they have not the right regard for looks. (si.com)
Reputable breeders limit the number of litters produced each year to ensure the health of the female cats and will take great care to avoid excessive inbreeding (damaging to the immune system) and to widen their gene pools. (messybeast.com)
The BBC produced two documentaries on dog inbreeding titled Pedigree Dogs Exposed and Pedigree Dogs Exposed — Three Years On that document the negative health consequences of excessive inbreeding. (breedingbusiness.com)