ZOOLOGY: T. H. MORGAN 
421 
the father), and that there are two of each kind of chromosome (one de- 
rived from the mother and one derived from the father). Moreover 
this relation becomes much more impressive when it is found that in the 
formation of the germ cells the representatives of the different charac- 
ters separate from each other, and that the chromosomes separate also, 
so that each germ cell has but one set of factors and but one set of chro- 
mosomes. The comparison does not stop even here, and I propose to 
review briefly the further evidence that leads to the conclusion that 
the chromosomes are the bearers of the hereditary characters and that 
the known chromosomal behavior suffices as a mechanism to explain 
MendeFs law. 
a b 
? c? 9 d 
Diploid Nuclei XX 
Gametes XXXO XXXY 
Fertilization 
Zygotes 
FIG. 1. DIAGRAMS OF SEX DETERMINATION; A, TYPE IN WHICH THE FEMALE IS XX 
AND THE MALE XO; 5, TYPE IN WHICH THE FEMALE IS XX AND THE MALE XY. 
Sex Linked Inheritance and the Sex Chromosomes. There is one ex- 
ception to the rule of the dual nature of the chromosomes and at the 
same time an exception to the rule of the dual nature of the hereditary 
factors. In many species of animals it has been shown that while there 
are two sex chromosomes in the female, there is only one in the male 
(fig. la). It is true in other animals often closely related to these, that 
the male may have one sex chromosome like the two in the female, and 
in addition a smaller Y chromosome, and in a few cases the Y may be as 
large as the X (fig. lb). In Drosophila there is an XY pair in the male, 
but the Y chromosome has been shown not to carry any factors that 
affect the characters due to factors carried by the X chromosomes. 
The influence of the Y is therefore nil, and the results are the same as 
though no Y were present. 
