INHERITANCE OF SEX AND RELATED PHENOMENA 205 
extra Y-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 Y-chromosome, therefore, must play some definite, 
positive réle 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 Y-chromosomes does not alter thisfact. If only one X-chromo- 
“some is present a male is produced, and it is immaterial whether no 
Y is present or whether one or two such chromosomes are present. 
Throughout, the inert nature of the Y-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 Y-chromosome. In non-disjunctional strains, 
however, some males are produced from the union of a Y-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 Y-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" LL 
eX | 
W Z ey 
| S | 
Wz 37 
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- 
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