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 7 , and one containing the 

 alternative factor for black by A" b . There are three kinds of female 

 cats in this respect, yellow (X 7 X y ), tortoise-shell (X y X h ), and 

 black (X h X b ). Males, however, having only one X chromosome, 

 can be of only two kinds, yellow (X 7 — ) 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 



