Genetics of Platypoecilus

ENETIC evidence of crossing over between the sex chromosomes of G heterogametic individuals (XU males and WZ females) is so rare in animals as to give special significance to additional data on this occurrence. AIDA (1921) has shown in the Japanese ricefield killifish, Aplocheilus, that the X and Y chromosomes of the male occasionally cross over. In 1930 he presented additional data on this, together with some interesting data involving aberrant sex ratios, which he interpreted as due to nondisjunction. In 1936 AIDA re-interpreted the abnormal sex ratios as due to sex reversal, following the suggestions of WITSCHI and WINGE. WINGE (1923, 1927) has shown in the West Indian guppy, Lebistes, that the X and Y occasionally cross over. In 1930 he stated that the genotypic chromosomal constitution of XX for femaleness in this fish may be overruled by other sex-determining factors so that the X X complex may occasionally produce a male. These XX males when mated to normal X X females produce, as might be expected, IOO percent daughters. This work he confirmed and extended in his papers of 1932 and 1934. FRASER and GORDON (1928, 1929) presented the first genetical evidence for crossing over between the W and Z chromosomes of a female Mexican platyfish (Plafypoecilus maculatus). BELLAMY (1933) accepted this interpretation and added several cases of his own. The data to be described in this paper increase the number of possible cases of crossing over between the W and Z from two to seven. A possible case (the first to be recorded) of crossing over between the two Z chromosomes of the male platyfish is also described. The new experiments are concerned chiefly with an analysis of the inheritance of a black banded pattern and its relation to other patterns. The variety with this pattern is known as the black platy. The symbol N (‘nigra’ or ‘niger’)was given to it by BELLAMY (1922).The history of domestication of this and other varieties of platyfish has been given by GORDON