JNHERITANCE OF SEX AND RELATED PHENOMENA 205 



extra F-chromosome has no influence on the determination of sex, 

 although it does give rise to unusual relations in the production of 

 gametes. Zygotes of the constitution XXX would presumably be 

 females, but they die and consequently nothing can be determined as 

 to their behavior. Males can be either normal XY or exceptional 

 XYY and XO. The last, although normal males in appearance, are 

 always sterile. The F-chromosome, therefore, must play some definite, 

 positive role in gametogenesis, although we are at present unable to state 

 just what its function is. Along with the preceding cases of female 

 constitutions, these different types of males indicate that the determina- 

 tion of sex depends upon the number of X-chromosomes present. If 

 two be present, a female is produced and the presence of one or two super- 

 numerary F-chromosomes does not alter this fact. If only one X-chromo- 

 some is present a male is produced, and it is immaterial whether no 

 F is present or whether one or two such chromosomes are present. 

 Throughout, the inert nature of the F-chromosome is emphasized, the only 

 evidence we have of its positive action being the sterility of XO males. 

 It is important also to note that the derivation of the chromosomes, 

 whether from the female or from the male, does not influence the sex 

 of the offspring. Ordinarily a male is produced when a gamete from the 

 female bearing an X-chromosome is fertilized by a gamete from a 

 male which bears a F-chromosome. In non-disjunctional strains, 

 however, some males are produced from the union of a F-bearing egg 

 with an X-bearing sperm, exactly the reverse of the usual procedure. 

 Also in such strains some females are produced by the union of an egg 

 containing two X-chromosomes with a F-bearing, or ordinarily 

 male-producing, sperm. Non-disjunction, therefore, establishes firmly 

 the intimate relation between chromosome constitution and sex 

 determination. 



The WZ Type of Sex-inheritance. A method of sex-inheritance 

 exactly the reverse of the XY type is that which Morgan has styled the 

 WZ type of sex-inheritance. In this type of sex inheritance the females 

 are heterozygous for a sex-determiner and the males homozygous. If 

 we diagram the relations which exist here, they will be as follows : 



WZ X ZZ 



I \ 



w z 



WZ 



The classical example of this type of sex-inheritance is Abraxas 

 grossulariata, and, as in the XY type, the evidence for the relations 

 obtaining in the inheritance of sex was given by the behavior of a sex- 



