ARTICLES ^ 591 



Unions producing males are indicated by dotted lines. 



Once this theory had been substantiated, investigations on 

 the subject proceeded along two distinct and definite lines. 

 Firstly, breeding experiments were instituted, and pedigrees 

 of families showing sex-limited characteristics were examined 

 with the object of tracing the inheritance of such points. 

 Secondly, cytological research was brought to bear on the 

 subject, and a physical sex factor was looked for as an elabora- 

 tion of the chromosome theory. This was, indeed, found as a 

 supernumerary pair of chromosomes ; or, at least, a pair in 

 one sex, and one — or one and a vestigial one — in the other sex. 

 The cytological aspect has been very fully worked out in 

 Drosophila — the American fruit-fly — and it has been con- 

 clusively shown that, with regard to this species, the male is 

 heterozygous, while the female is homozygous. For obvious 

 reasons, cytological research has not yet been pushed to any 

 length in the higher animals, including man, and any evidence 

 we have on these has chiefly come from breeding experiments 

 and reconstructed pedigrees. The higher animals are, however, 

 very awkward subjects for breeding experiments, and little has 

 been done with anything larger than rabbits. The higher the 

 animal the more awkward it is, because of the fewness of the 

 offspring and the long periods of immaturity and gestation. 

 With man, the research worker is reduced to family records 

 and published genealogies, which are not very plentiful, 

 especially when abnormalities are under consideration. In 

 spite, however, of these obstacles, enough has been done to 

 place man and the higher animals with Drosophila in possessing 

 heterozygous males and homozygous females. 



Breeding experiments with butterflies and moths, particu- 

 larly with the currant moth, and with domestic birds point to 

 directly opposite conclusions ; that is to say, that the male is 

 homozygous and the female heterozygous. For the present, 

 therefore, we have no choice but to assume that at least two 

 types of sex determination obtain — the one exhibited by Aves 

 and Lepidoptera, and the other by Drosophila and Mammalia. 



The evidence which places man among the latter is, as 

 mentioned above, chiefly obtained from pedigrees showing sex- 

 limited characteristics. Among the more well known of these 

 may be mentioned haemophilia and colour-blindness. A some- 

 what diffuse characteristic, which does not seem to have received 

 much attention, is the numerical preponderance of one sex over 

 another in a family. The writer, having occasion to want 

 information on the subject in an agricultural connection, was 

 unable to find any details with regard to this phenomenon, and 

 this paper is the result of an effort to gain a slight amount of 

 information on the subject. 



