Balance Theory of Sex 481 



ANIMALS 



Sex Chromosomes 



The relation of chromosomes to sex determination was pointed 

 out in Chapter 5. In Drosophila, human beings, and other mam- 

 mals, the female has two X chromosomes and the male is XY, 

 but in some animals the male has one X chromosome but no Y. 

 In moths, butterflies, birds and some fish the female is the 

 heterogametic sex. There is some question how the sex chromo- 

 somes in animals of this type should be designated. Originally, 

 the sex chromosomes of the female were designated Z and W, and 

 the male was said to be'ZZ. Reasoning that it is unnecessary 

 to use the additional symbols Z and W when X and Y can be 

 used for the sex chromosomes in all organisms irrespective of 

 whether the male or the female is heterogametic, some geneticists 

 use the symbols X and Y for the sex chromosomes in the female 

 of animals in which the female is heterogametic and XX for the 

 males of such forms. Thus the symbols are the same as those 

 of the Drosophila except that the sexes are reversed with respect 

 to the chromosome constitution of the sex chromosomes. Castle, 

 however, has suggested that X be used to indicate a sex chromo- 

 some with a female tendency and Y one with a male tendency. 

 If this scheme is carried out, Castle points out that in the 

 Abraxas or bird type the female should be XY and the male 

 YY. Although the choice of symbols is largely a matter of pref- 

 erence, the use of both XX and YY by different geneticists for 

 the male with the accompanying reversal in meaning of the "X" 

 and ''Y" in the female of moths and birds is a little confusing. 

 For that reason, largely, the older system of ZW and ZZ has 

 been adopted in this book. 



Balance Theory of Sex 



With a mechanism such as the sex chromosome mechanism in 

 Drosophila melanog aster, it is possible that maleness might be 

 determined by the presence of a Y chromosome or that female- 

 ness migjit be determined by two X chromosomes. So far as the 

 evidence from diploid organisms is concerned, either possibility 

 is philosophically sound. Further evidence from organisms which 

 are polyploids or aneuploids, however, indicate that the second 



