290 Shull. 



Cj'tological studies have shown iu a considerable nunil)er of animals 

 belonging to several different natural groups , that the female contains 

 a pair of special chromosomes, the "X- chromosomes", while the male 

 has onlj' one similar chromosome, either unpaired or paired with a chro- 

 mosome which is often smaller than the X- chromosome, and wliich is 

 known as the "Y-chromosome". Sometimes instead of the Y-chromo- 

 some there is a group of chromosomes which remain associated together 

 as a "Y- element", this group being included, as a whole, in 50 per cent 

 of the sperms, the X- chromosome being present in the rest. 



These results have led to the rather frequent adoption of the 

 letters X and V in the genotyjiic formulations of the sexes, but there 

 appear to me to be certain more or less serious disadvantages of such 

 use. It was entirely proper that Castle (1909) should use the formu- 

 lae, XX = o, X or XY — c?, in assisting Wilson (1909) to clear up 

 his difficulties iu aligning the cytological observations on the "X"- and 

 "Y"-chromosomes, with current interpretations of Mendelian inheritance; 

 but now that these difficulties have entirely disappeared, there seems to 

 be no good reason why the earlier method which represents the deter- 

 miner for femaleness )jy F and its absence by f, and the determiner for 

 maleness by 1/ and its absence by w, should not be universally followed 

 in constructing the genetic formulae of the sexes. These symbols are 

 in entire accord with the principles of standardization suggested above. 



Besides the X- and Y-chromosomes we now have references in the 

 literature to the "Second" and "Third" chromosomes. There has been 

 established in tliis way a rather definite chromosome terminology whose 

 advantages are manifest and which should be made permanent by con- 

 sistent usage. The relations between the Mendelian genes and the 

 chromosomes are still a matter for speculation and discussion, and just 

 because these relations must doubtless long remain problematic, it seems 

 inadvisable to continually imply by the symbols, that chromosomes and 

 Mendelian genes are synonymous terms. The inadvisability of the use 

 of A' for the sex-gene is rendered more obvious by the apparent demon- 

 stration, through the brilliant work of Morgan (1911c) with Drosopliila, 

 that the sex-determiner is probably not the X- chromosome, as a whole, 

 but only a part, — perhaps a very small part, — of it. We should be able 

 to say without tautology that the sex-determiner (F rather than X) is 

 probably carried by the X-chromosome. 



The difficulties due to the introduction of chromosome-terminology 

 into genotypic formulae are accentuated when the symbol Y, representing 

 a gene for vellow i)igmentation, is introducod into the same formula in 



