26 BULLETIN 905, U. S. DEPARTMENT OF AGRICULTURE. 
which the blood fails to clot properly, with the consequence that 
an affected individual may bleed to death from a slight scratch. 
Until recently the explanation of this kind of inheritance was a 
complete mystery. A little consideration, however, will show that 
it is exactly what should be expected of a trait due to a factor carried 
by the chromosome which determines sex. The evidence indicates 
that in the human species this so-called X chromosome is single in 
males and paired in females. Half of the sperm cells produced by 
a male contain the X chromosome and hence can transmit hereditary 
traits which it contains. These are the female-determining sperms. 
The other half, the male-determiners, lack the X chromosome and 
all that is transmitted by it. This explains why a sex-linked charac- 
teristic can not be transmitted from father to son or to any of the 
descendants of the latter. All the daughters of an affected male 
receive the abnormal factor in the X chromosome which determines 
their sex. They will not, however, show the abnormality themselves, if 
it is recessive, as they also in general receive a normal X chromosome 
from their mother. Half of their ova, however, will contain the 
affected X chromosome. Those fertilized by sperms which lack 
the X chromosome are sons, and they will show the abnormality, 
as they can receive no protecting normal factor from the father. 
The common yellow variation of cats is another example of a 
characteristic which is linked with sex in this way, except for the 
fact that dominance is lacking. We may represent an X chromo- 
some containing the factor for yellow by X y , and one containing the 
alternative factor for black by X b . There are three kinds of female 
cats in this respect, yellow (X y X y ), tortoise-shell (X y X h ), and 
black (X h X h ). Males, however, having only one X chromosome, 
can be of only two kinds, yellow (X y — ) and black (X b — ). It has, 
in fact, long been known that tortoise-shell males are so rare as to 
be classed as freaks. It should be added that there are independent 
factors which may change black in either sex to maltese or the tabby 
pattern. 
The female-determining sperm cells contain the extra chromosome 
(X y in the case of a yellow male, X h in the case of a black male). 
The male-determining sperm cells wholly lack this chromosome. 
Thus a male can have no influence on the color of his sons so far as 
this pair of factors in concerned. A black female produces only 
black sons, a yellow female produces only yellow sons, and a tortoise- 
shell female produces 50 per cent of each on the average regardless of 
the color of the male in each case. A male can, however, transmit 
his color to the sons of his daughters. 
In an earlier section it was noted that Prof. Morgan and his co- 
workers had found that the hereditary variations of the fruit fly 
fall into four linkage groups, corresponding to the four pairs of 
