The am Locus

Symbol: am
Also known as: Amputated locus
Sources: Hollander and Miller 1978: “Amputated, frayed, and sideburns; three new mutants in the pigeon”

Pigeon expressing amputated, from the Hollander and Miller study.

Function

A locus associated with a variable congenital reduction of distal structures, most commonly affecting the toes/claws, sometimes the digits of the wing, and more rarely the lower beak.

Alleles at this locus

  • am – Amputated (mutant)
  • am+ – Wild type (normal)

Inheritance

Autosomal recessive, with reduced penetrance / variable expression.

Key breeding observations in the Hollander and Miller paper:

  • Matings of defective birds to normals produced 30 F1, all normal, and sex linkage was considered excluded (e.g., daughters of an abnormal male were normal).
  • The authors treat it as simple recessive inheritance, but note that the results depart from ideal Mendelian expectations, consistent with reduced penetrance and/or classification ambiguity in mildly affected birds.

Primary genetic effects

Most common presentation: the inner toe (second digit) of one or both feet lacks the claw and may be reduced to a stump; severity ranges from very mild to affecting most toes.
More severe presentations: abnormalities of most/all toes, reduction of the digital portion of the wing, and occasionally shortening/defect of the lower beak (including at least one very abnormal young that could not hatch due to only a rudiment of the lower beak).

Genetic interactions

No specific epistatic interactions are established. The authors suggest that genetic modifiers and environmental variables (especially incubation conditions) may influence expression/variability, and also note that some mildly affected birds may have been misclassified as normal in early tests.

Linkage

No known linkages.

Associated genes / alleles

  • Amputated (am)

Phenotypes influenced

  • Amputated

Notes and uncertainty

  • The authors explicitly state it remains to be determined whether the mutant reflects true “amputation” of embryonic structures vs growth inhibition, pending further embryological study.
  • Expression is described as wide and seemingly erratic, consistent with reduced penetrance/variable expressivity.