T H E Production of a Melanotic Neoplastic Disease I N Fishes by Selective M.4tings Iv. Genetics

When a black-spotted female platyfish (Platypoecilus maculatus) is mated to an unspotted (genetically recessive) male platyfish, one of three hereditary expressions may be expected. If the female is genetically pure (homozygous dominant) for the spotted factor, all of the young may be expected to be spotted. The genetic formula representing this situation is (W)Sp(Z)Sp, for the mother, where W and Z represent the sex chromosomes upon which the two hereditary factors (Sp, for spots) are carried. The recessive father’s constitution is (Z)sp(Z)sp. When another type of spotted female is mated to a similar unspotted male platyfish, only her sons are spotted, like their mother; the daughters are unspotted, like their father. This is the well known type of criss-cross inheritance and indicates the presence of sex-linked factors. The formula that represents the spotted mother of this type is (W)sp(Z)Sp where the dominant factor S p is carried on the Z chromosome and the recessive s p is on the W. The father’s formula is (Z)sp(Z)sp, the same as before. When still another type of female spotted platyfish is mated to an unspotted platy male, (Z)sp(Z)sp, the following results are obtained: all the daughters are spotted like their mother while all the sons are spotless like their father. In this case, analysis shows that the spotted mother’s constitution is (W)Sp(Z)sp where the dominant factor for spots S p is carried on the W chromosome. The \V chromosome presumably carries hereditary factors potent for the development of the female sex. The origin of these three types of spotted females is described by Fraser and Gordon (1929). Spotted female platyfishes (Platypoecilus maculatus) have been mated to males of a different species and genus, the Mexican swordtail (Xiphophorus hellerii) ; the spotted character of the mother appears in her hybrid offspring. If the mother’s constitution is (W)Sp(Z)Sp, the spotted character appears in all the offspring; if the mother is (W)sp(Z)Sp, it appears only in the sons; if the mother is (W)Sp(Z)sp, the spotted character appears only in the daughters. Only rarely do exceptions appear. The subject is reviewed by Gordon (1931, 1937). In the black spotted hybrids resulting from these crosses the pigment cells, which form the spots, grow at an abnormal rate. At birth there is some immediate indication of accelerated growth of the large black pigment cells.