172 the chromosomes 



Attachment of Sex-Chromosomes to 

 Autosomes 



If one or more genes for sex are present in a 

 chromasome, that chromosome would be designated 

 as a sex chromosome, but in order that the mecha- 

 nism give a differential result such chromosome 

 must be present in duplex (XX or ZZ) in some in- 

 dividuals and simplex (X or Z) in others. It is not 

 necessary in the simplex individual that one whole 

 chromosome be absent, but only that one should lack 

 factors for sex, as in the case of Y-chromosome 

 in one type, and supposedly the W-chromosome in 

 the other. 



There is still another situation in regard to the 

 chromosomes that carry the sex genes. In a few 

 cases there is more than a suspicion that they are 

 attached to other chromosomes, or what amounts 

 to the same thing,that the region of the chromosomes 

 bearing sex determining genes lies at one end of an 

 ordinary chromosome. In such cases two of these 

 chromosomes with attached portions are supposed 

 to be present in the female (the XX type), but in 

 the male only one chromosome of the pair has the 

 attached portion. The thread worm, Ascaris, is a 

 case in point. It has been observed by Boveri, 

 Boring, Frolowa, Mulsow, Kautsch, and Geinitz 

 that occasionally one or two small detached pieces 

 of chromosomes are found constantly present in 

 certain individuals — pieces that show a tendency 

 in certain cells to become attached, or associated 



