EEVIEW OF BIOLOGY OF SEX-DETERMINATION 
11 
female brood, but these he explains on the basis of an imperfect 
mitosis, resulting probably in the loss of a specific chromosome 
which probably bore the sex-determining genes. This supposi- 
tion is based on direct cytological observation. The facts of 
polyembryony offer a strong substantiation to the idea of 
chromosomal determination of sex. 
5. SEX-LINKED INHERITANCE 
The association of Mendelian characteristics with particular 
chromosomes is nowhere better shown than in the group of the 
sex-linked characteristics. The genes for these characteristics, 
of which alone thirty odd are known for Drosophila , are un- 
doubtedly located in the sex chromosomes, and their inheritance 
follows the distribution of these chromosomes exactly. 
Let us take for example the inheritance of red eye in Droso- 
phila, a dominant sex-linked characteristic (see fig. 4). If a 
red eyed female is mated to a white eyed male, the are all red 
eyed. If the F^ are again inbred, the F^ generation are three 
red eyed to one white eyed, but the peculiar thing is that all the 
white eyed individuals are males. Thus, half the F^ males are 
like their grandfathers. White-eyedness is covered up when the 
gene for red is present, and this is the case in all the F^ females. 
However, the eggs of the F^ female, which eliminate the red gene 
in the polar body in maturation and are then fertilized with a 
sperm bearing a Y-chromosome, will result in white eyed offspring. 
Thus we can say that the males have inherited their white eyes 
from their mothers through the X-chromosomes which she con- 
tributed to the zygotes. 
Let us now examine the cross reciprocal to that given first. 
As shown in figure 5, we get an entirely different result. The 
F^ females are red eyed like their fathers, and the males are white 
eyed like their mothers. In the F^ generation half the males 
and half the females are white eyed and the others red eyed. 
This result is due to the fact that the male has a mechanism 
(only one X-chromosome) capable of bearing the gene for red 
but once. This is a cross therefore of a heterozygous dominant 
