Similarly, if you mix two animals that have close genetic makeups, the offspring won’t be as healthy as the offspring of two genetically dissimilar parents. That’s because many diseases are caused by mutant genes. If you inherit a mutant gene from your mother, you can use the non-mutant version inherited from your father as a backup. If both mother and father had mutant versions of the gene, however, disease will result. Inbreeding makes it much more likely that two mutant copies of a gene will show up. So you can see the problem with endangered species. With fewer than about five hundred members, the gene pool is too restricted. Inbreeding is more likely. The following generations become progressively less healthy, eventually dying out.