Mendelian trait/disorder: yes

Mode of inheritance: Autosomal Recessive

Considered a defect: yes

Key variant known: no

Species-specific symbol: ac-2

History: This form of disproportionate dwarfism was first reported by Crary and Sawin (1952), who suggested the symbol 'da' (without any explanation) for the mutant recessive gene. Also without explanation, Sawin and Crary (1959) and Sawin et al. (1959) referred to this mutant as the "dachs" mutant, possibly as an indication of similarity with the disproportionate dwarfism of dachshund dogs. These authors also reversed the notation of the alleles, referring to the mutant allele as 'Da' and the wild type as 'da'. Their data, however, were more consistent with the mutant being recessive. Confusingly, Robinson (1958) used the symbol 'ac-2' for this mutation, and allocated the symbol 'da' to Dwarfism, Dahlem (OMIA 001998-9986).

Inheritance: Sawin and Crary (1959) reported many body measurements that suggested a relatively small effect of the mutant gene in heterozygotes.

Clinical features: Sawin and Crary (1959): "The homozygous dachs manifests certain of the typical characteristics of chondrodystrophy but there are certain unique differences. Diminished size and shortening of the limbs appear to be consistent but variable in their expression. Hyperplasia of cartilage in the region of the metaphysis and any profound enlargement of the head seem to be absent except as they may be secondary manifestations or are mildly expressed by individual bone measurements. The depression at the nasion appears to have migrated posteriorly and is accompanied by depression and shift of the orbit and optic foramen. Associated with these are highly localized disproportionate differences which are unlike those noted in any other inherited chondrodystrophy thus far described."

As summarised by Robinson (1958, p. 341), "The present case differs from . . . [Achondroplasia-1; OMIA 001996-9986] in several respects. The animals are fully viable and appear quite normal at birth but when adult have the characteristic short legs. Externally the achondroplasia can be recognised by a small cartilaginous papilla arising at the base of the ear. This is present at hirth but is not fully detectable until the ear flap opens at approximately the sixth day and the papilla separates from the remainder of the ear. X-ray plates reveal abnormalities of the leg bones which at a later age may result in crippled legs and a peculiar undulating gait. Dislocations of the hip or knee may appear and become more pronounced as maternity is reached, interfering with normal and successful mating. However, it has been possible to produce young from one [homozygous] female by a normal male. Ear carriage is modified so that it tends to project downward and outward instead of upward and backward. SAWIN (1955) notes that the heterozygote . . . possesses a remnant of the papilla in the same relative position at the ear base."