Sex in Relation to Chromosomes and Genes 



CALVIN B. BRIDGES 



Reprinted by publisher's permission from Amer- 

 ican Naturalist, vol. 59, 1925, pp. 127-137. 



It sometimes happejis in biology that a hypothesis that fits all 

 knoiv?! facts a?id evidence is offered and accepted for many years. 

 It may prove to he so completely satisfactory that it becomes ac- 

 cepted as fact rather than as hypothesis. This was true of sex deter- 

 ?ninatio?i, which was lojig understood to be a simple matter of the 

 presence or absence of a particidar chromosome. In maimjials a?id 

 other groups it ivas a "F" chroinosome^ which, presefit and paired 

 with an "X" chromosome, gave a male. If the Y was absent, and two 

 X chromosomes were paired, a female was born. The presence and 

 absence theory was sufficient to explain sex detertnination in many 

 different kinds of platits and animals. 0?ily the occurrejice of a situa- 

 tion the cojicept could ?iot explain led Bridges to fonmilate a differ- 

 ent theory— that of chromosome balance. 



We still do not know today how widely the balance theory of 

 sex deter?nination ca?i be applied. Bridges shows in this paper that it 

 might apply i?i the case of several pla?it species, but this is primarily 

 on the strength of extrapolated evideyice. But Tnany geneticists today 

 believe that the hypothesis of the balanced chromosome determina- 

 tion of sex in the fruit fly is equally valid throughout a wide range 

 of species, a?id this paper sta?ids as a classic i?i the study of sex de- 

 terminatio?!. 



During the three years since X-chromosomes and three of each 



the report at the Toronto meeting kind of autosome. The possession of an 



(Bridges '22) considerable new in- extra X and at the same time of an 



formation has accumulated with re- extra set of autosomes does not change 



gard to the series of different sex-types this individual in its sexual characters 



that has arisen in the breeding work from the normal type of female. How- 



WTth Drosophila (Table I). Each of ever, in gametogenesis the 3N group 



these different sex-types is the result is an unstable one. Each egg receives 



of a particular combination of chro- a full set of chromosomes and a full 



mosomes. They occur principally set goes to the polar body. The mem- 



among the offspring of females that bers of the extra set are distributed 



are triploids, that is, that have three between the egg and the polar body 



117 



