Inbred animals are likely conduits for certain specific characteristics coming from their inbred ancestor. Inbred strains are also available but are not as commonly used as inbred mice. Inbreeding avoidance mechanisms have evolved in response to selection against inbred offspring. Mouse strains can be inbred, mutated, or genetically modified, while rat strains are usually inbred. There are hundreds of established inbred, outbred, and transgenic strains. MHC genes were first identified in inbred mice strains. This maintains genetic variability in a species that is otherwise highly inbred. A high population of puppies from mills are inbred due to uncontrolled breeding. Another source suggests 1,000 inbred domesticated strains are kept worldwide. An individual who inherits such deleterious traits is colloquially referred to as "inbred". They were inbred to Snake in the fourth generation of their pedigree. Recent studies have demonstrated that motherly behavior is not inbred but biologically and socially determined. The mice that are inbred typically show considerably lower survival rates. Many (but not all) laboratory strains are inbred. At that time, all the bloodlines were closely inbred with each family clan.