Gene pool

Because relatives tend to share many of the same alleles, continued inbreeding can reduce the variety of alleles in the population: its gene pool becomes smaller.

Repeatedly breeding from a limited group of plants can narrow the crop's gene pool. A genetically uniform crop may perform very well under familiar conditions, but if a new pathogen arrives and most plants share the same susceptibility, much of the crop could be affected. Reduced genetic variation also leaves breeders with fewer alternative alleles to use when conditions change.