Overview of Axolotl Genetics and Nomenclature
Axolotls have 28 chromosomes (Fankhauser and Humphrey, 1942). Signoret (1965), Callan (1966), and Cuny and Malacinski (1985) have described the karyotype. The female axolotl is heterogametic (Z/W) and the male homogametic (Z/Z) (Humphrey, 1975).
A variety of mutant genes have been identified in axolotls. Many of these are carried by the Axolotl Colony stocks. The most obvious are those which determine pigmentation or coloration of the axolotl. Others affect organs (eyes or heart), limbs, or gills.
Axolotls are diploid; thus they carry two copies of each of their genes. Each copy of a gene is called an allele. If both alleles are the same, the axolotl is homozygous with respect to that particular gene. Mutations, which give rise to different alleles, may be dominant or recessive. A dominant allele is expressed (the animal displays the trait) even if the axolotl is heterozygous for that gene and carries only one copy of the allele. A recessive allele is not expressed unless the axolotl is homozygous for that gene and carries two copies of the mutant allele.
In our system of notation, the symbols for the alleles of a gene are written on either side of a slash. A dominant allele is written with a capital letter or with a plus sign (+). a recessive allele is written with a lower case letter. For instance, an animal with the genotype D/d m/m is dark, because D is a dominant gene for the dark, wild-type axolotl color, and melanoid (without yellow mottling), because it is homozygous for the recessive melanoid (m) gene. It also carries, but does not express, the gene d (white). An animal with genotype d/d +/m displays the white phenotype, because it is homozygous for the gene d. It carries, but does not display, the melanoid trait. The + represents the dominant allele (it could be written M), in this case the wild-type, nonmelanoid phenotype.
Several overviews of the axolotl mutants have been published, e.g., Malacinski (1978), Armstrong (1985), and Malacinski (1989).