Referate. gc 



J. H. Gerould, The inheritance of Polymorphism and Sex in Colias philodice. 



American Naturalist, Vol. 45, No. 535. May igri p. 257 — 283. 



This interesting paper describes breedinj,' experiments with Colias 

 philodiie, a N. American butterfly which like the Euro]iean C. ediisa has 

 the male with a yellow ground-colour, and a dimorphic female. The 

 commoner form of female has a ground colour like that of the male, but 

 a variety occurs in which the colour is nearly white. This form varies in 

 frequency from about 5 per cent in some places to nearly 50 per cent, in 

 others. In localities in which the pale female is common, pale males occur 

 very rarelj', but the author has not obtained one to breed from, and none 

 appeared in his cultures. A very rare melanic form of the male is also 

 known, and the author obtained a dark aberration of the pale female, 

 resembling the arctic C. nastcs, possibly the effect of cold upon the pupa, for 

 the offspring of this female were normal yellow or white. 



The breeding experiments, the results of which are given in tables, 

 lead to the following conclusions 



The white female (13 tested) is always heterozygous for colour, 

 producing, when mated with a yellow male, either equal numbers of white 

 and yellow females, or twice as many whites as yellow. All the males in 

 every case are yellow. These results lead to the conclusion that white is 

 dominant in the female, but yellow dominant in the male. When a white 

 (heterozygous) female is crossed with a heterozygous male, the ratio of 

 white to yellow among the female offspring is not 3:1, as would be the 

 Mendelian expectation, but about 2:1. This is explained by the hypothesis 

 that homozygous whites cannot be formed, the case being similar to that 

 of the yellow mouse in which homozygous yellow does not occur, with the 

 result that yellows bred together give two yellows to one non-yellow. This 

 hypothesis also explains the absence of white males; homozygous whites 

 canncit be produced, and in the male the heterozygote is yellow. Yellow 

 females are presumably homozygous, and when mated with homozygous 

 yellow males give only yellow offspring When mated with heterozygous 

 males, usually white and yellow females are produced in equal numbers, 

 but in one case 30 white and 14 yellows appeared; the excess of whites in 

 this case, and the excess of yellows in some broods where equality was 

 expected, suggest that some yellow females may be heterozygous. 



In a comparison with the few observations that have been made with 

 Colias edusa, it is shown that the pale variety {hclici-) is probably similar in 

 its behaviour to the pale form of C. philodice, although the large excess of 

 pale females reared from a pale parent in one case suggests that in C. cdusa 

 homozygous pale females may possibly occur. In a few general observations 

 in the genus Colias it is suggested that the female type of pattern is 

 probably more primitive, the male being more highly modified. 



The author then proceeds to discuss the inheritance of sex. He regards 

 the male as being homozygous for a sex-determiner, ('x'), the female 

 heterozygous (male xx, female xo). This factor x he speaks of as 'dominant', 

 a some what unfortunate use of the word, since it is supposed to be present 

 in the female (xo) without producing visible effect, and is only effective 

 when homozygous (xx) in the male. The yellow and white ground colour 

 are due to a pair of allelomorphs y (yellow) and its absence (w = white). 

 Males may thus be yyxx or ywxx, both being yellow in colour; females 

 may be yyxo (yellow), or ywxo (white, since in the female only homozygous 

 y gives yellow colour.) If then the combinations wwxx and wwxo are 

 mfertile, the observed results will be obtained. 



