Tailless RatsNote: I am a biologist, not a rat breeder, therefore, this is not a section about tailless rats as pets. This is an article about tailless rat biology.There are at least two mutations that cause taillessness in rats: Tail-anomaly lethal (Tal) and stub (st).
• Tail-anomaly lethal (Tal): The autosomal dominant mutation Tal is lethal in its homozygous state (when the rat gets two copies of the gene): Tal/Tal embryos die between days 9 and 10 of gestation (Hoshino et al. 1979). In heterozygotes, (the rat has one Tal copy and one normal copy of the gene), Tal causes taillessness.
Tal/tal heterozygoes vary widely: tail length ranges from zero to normal, and the number of kinks in the tail ranges from one to six per tail, and the kink severity ranges from barely perceptible to complete hooks and corkscrews. Tailless rats also have lower body weight than normal rats (Schaid et al 1982, Hoshino et al 1979). This size difference between normal and tailless rats is greater when taillessness came from the mother than the father, possibly because the tailless mother's small size affects her ability to rear her young. Some heterozygotes have normal tails yet can produce tailless offspring, thus taillessness may skip generations (Schaid et al 1982).
This variation in phenotype indicates incomplete penetrance of the Tal mutation. Genetic penetrance -- the liklihood that an individual carrying a mutation will develop the characteristics caused by that mutation -- is estimated to be between 81.8% and 90.6% (Schaid et al. 1982).
• Stub (st): The second mutation known to affect tail length is the recessive mutation stub. Homozygous rats (st/st) usually die at birth (Ratcliffe and King, 1941), indicating that stub's action is different from that of Tal. The stub mutation is now believed to be exinct within the population of laboratory rats (Schaid et al. 1982).
Tailless rats and thermoregulation
The rat's tail helps to regulate its body temperature. Tailless rats do not lose heat as well as tailed rats -- their core body temperatures rise higher and it takes longer for their high temperatures to come down. For example, Spiers et al. (1982) found that when tailless and tailed rats were warmed, the body temperature of tailless rats rose 0.2-0.5 degrees higher than that of tailed rats. Afterwards, it took the tailless rats nearly an hour longer to cool down to a normal temperature than the tailed rats.
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