INTRODUCTORY. 7 



from the male. Many other combinations show the same relations. 

 In the case of non-disjunction, to be given later, there is direct experi- 

 mental evidence of such a nature that there can no longer be any 

 doubt that the X chromosomes are the carriers of certain gens that 

 we speak of as sex-linked. This term (sex-linked) is intended to mean 

 that such characters are carried by the X chromosome. It has been 

 objected that this use of the term implies a knowledge of a factor for 

 sex in the X chromosome to which the other factors in that chromosome 

 are linked; but in fact we have as much knowledge in regard to the 

 occurrence of a sex factor or sex factors in the X chromosome as we 

 have for other factors. It is true we do not know whether there is 

 more than one sex-factor, because there is no crossing-over in the 

 male (the heterozygous sex), and crossing-over in the female does not 

 influence the distribution of sex, since like parts are simply interchanged. 

 It follows from this that we are unable as yet to locate the sex factor 

 or factors in the X chromosome. The fact that we can not detect 

 crossing-over under this condition is not an argument against the 

 occurrence of linkage. We are justified, therefore, in speaking .of the 

 factors carried by the X chromosome as sex-linked. 



CROSSING-OVER. 



When two or more sex-linked factors are present in a male they are 

 always transmitted together to his daughters, as must necessarily be 

 the case if they are carried by the unpaired X chromosome. If such 

 a male carrying, let us say, two sex-linked factors, is mated to a wild 

 female, his daughters will have one X chromosome containing the 

 factors for both characters, derived from the father, and another X 

 chromosome that contains the factors that are normal for these two 

 factors (the normal allelomorphs). The sons of such a female will get 

 one or the other of these two kinds of chromosomes, and should be 

 expected to be like the one or the other grandparent. In fact, most 

 of the sons are of these two kinds. But, in addition, there are sons 

 that show one only of the two original mutant characters. Clearly 

 an interchange has taken place between the two X chromosomes in the 

 female in such away that a piece of one chromosome hasbeen exchanged 

 for the homologous piece of the other. The same conclusion is reached 

 if the cross is made in such a way that the same two sex-linked characters 

 enter, but, one from the mother and the other from the father. The 

 daughter gets one of her sex chromosomes from her mother and the 

 other from her father. She should produce, then, two kinds of sons, 

 one like her mother and one like her father. In fact, the majority of 

 her sons are of these two kinds, but, in addition, there are two other 

 kinds of sons, one kind showing both mutant characters, the other kind 

 showing normal characters. Here again the results must be due to 

 interchange between the two X's in the hybrid female. The number of 



