The sickle tail is the single
phenotypic
In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology (physical form and structure), its developmental processes, its biochemical and physiological propert ...
characteristic that seems to separate dogs from wolves.
Dogs who have tails tend to have an upward curve in the tail, which is called the sickle tail. Wolves' tails, on the other hand, hang straight; this is called the brush tail—similar to that of a fox.
References
Dog anatomy
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