Rumpless Type | Dominant Complete | Dominant Incomplete | Recessive | Accidental |
Anatomical Differences from Normal Tail | Outward Appearance |
No tail or tail feathers. No outward visible difference between complete genetic and accidental rumpless. Difficult to distinguish types by dissection. | More variable than other types. |
Known as "roachback". Fused caudal vertabrae often bend downward and appear laterally compressed. Some birds show curvature of the spine and humped backs. |
No tail or tail feathers. No outward visible difference between complete genetic and accidental rumpless. Difficult to distinguish types by dissection. |
Synsacro-caudal Vertabrae | One or two vertebae missing from center of the 5 synsacro-caudal vertebrae. | One or two vertebrae missing from center of the 5 synsacro-caudal vertebrae. | No missing synsacro-caudal vertebrae. | Two (rarely one) vertebrae missing from end of the 5 synsacro-caudal vertebrae |
Bony Knob | Small bony knob on last vertebra (caudal end of vertebral column ). | Small bony knob on last vertebra (caudal end of vertebral column). | Pygostyle and free caudal vertebrae commonly fused into a bony knot. | No small bony knob on last vertebra (caudal end of vertebral column) |
Pygostyle | Some consider the small bony knob on last vertebra homologous with the last two pygostyle vertebrae. | Some consider the small bony knob on last vertebra homologous with the last two pygostyle vertebrae. | Sometimes absent, more commonly fused with free caudal vertebrae into a bony knot. | No pygostyle |
Uropial Gland | No uropial gland | 75% no uropial gland, less than 2% functional uropial gland | Absent or rudimentary | No uropial gland |
Free Caudal Vertebrae | No free caudal vertebrae | Distinguishing feature: all five caudal vertebrae usually present but irregularly fused together. | Free caudal vertebrae commonly fused with pygostyle into a bony knot | No free caudal vertebrae |
Retrices | No retrices | 2-17 retrices | 0-17 retrices | No retrices |
Supernumerary Ribs | None | None | Up to 17% of males and 8% of females have supernumerary ribs, mostly rudimentary, on one or both sides | None |
Genetics | ||||
Genetic Basis | Genetic | Genetic |
Genetic Environmental, not transmitted to progeny. Caused by accidents when embryo developed | |
Dominant or Recessive | Dominant | Dominant | Recessive | Not genetic |
Genetic Symbol | Rp | Rp | rp 2 | None |
Modifying Genes | Sometimes unaffected by modifying genes | Modifying genes can cause incomplete rumplessness even in birds homozygous for Rp | Strongly influenced by modifying genes. Low penetrance , only 33 to 38% of the offspring from two recessive rumpless birds are rumpless | Not genetic |