214 



JSCIENCE 



[N. S. Vol. LIV. No. 1393 



"typical Mendelian," etc. All such . qualify- 

 ing expressions give evidence of the recogni- 

 tion of the fact that usage varies regarding 

 the significance of the words " Mendelism " 

 and " Mendelian." 



Since there are these differences of usage 

 among geneticists, it would seem to be 

 necessary for any one who describes a geneti- 

 cal situation as Mendelian or non-Mendelian, 

 to state just what meaning is to be attached 

 to the expression he uses. In my own usage 

 of the expression " Mendelian heredity " it 

 has always referred to cases such as Mendel 

 actually observed, in which there is (statisti- 

 cally) iadependent segregation of unit factors 

 during gametogenesis and chance recombi- 

 nations at fertilization. I had this conception 

 in mind in declaring'' that the genetical 

 phenomena in the Oilnotheras are, with rare 

 exceptions, non-Mendelian. 



As Mendel never observed a case of linkage 

 and no provision is made for such a phenome- 

 non in the theory by which he interpreted his 

 results, such cases are, on this basis, to be 

 considered non-Mendelian, — especially as they 

 definitely contradict the fundamental Mende- 

 lian postulate of independent segregation. 

 This may perhaps with some justice be termed 

 the strict-constructionist view. On the other 

 hand, since it is now obvious that strictly 

 Mendelian phenomena and linkage phenomena 

 are products of the same mechanism and 

 indeed that linked genes are in many cases 

 quite indistinguishable from wholly indepen- 

 dent ones, there is some justification for those 

 who give a broader construction to the term 

 Mendelian, making it essentially synonymous 

 with chromosomal heredity as distinguished 

 from cytoplasmic heredity. 



In view of these discrepancies in usage by 

 different authors, has not the time come to 

 abandon the use of " Mendelian " and " non- 

 Mendelian " as definite categories, and to 

 adopt other terms which will have greater 

 precision of meaning? It seems to me that 

 the accumulation of facts from genetical 



« Shull, G-. H., ' ' A peculiar negative correlation 

 in CEnotliera hybrids," Jour. Genetics, 4: 83-102. 

 1914. 



investigations has reached such magnitude 

 as to justify an attempt in this direction. 



In offering a terminology for several of the 

 fundamental categories of genetical phenom- 

 ena my object is chiefly to emphasize by 

 this means the fact that the categories them- 

 selves do exist and that they have been (and 

 are) recognized by geneticists. 



Very few (if any) geneticists will now fail 

 to agree that the relation of hereditary 

 factors to linkage groups, or to paired pa- 

 ternal and maternal material bodies, the chro- 

 mosomes, must provide the basis for such a 

 classification. Since we have long been 

 familiar users of two words, homozygous and 

 heterozygous, derived from the Greek root 

 ^vy- (^evyvv/ii, to join, ^ev^is, a yoking; t,vy6v, 

 a yoke), it seems appropriate to use the same 

 Greek root as the basis of the more complete 

 terminology here suggested. 



To distinguish between phenomena which 

 are dependent upon the distribution of the 

 chromosomes, and those phenomena which are 

 to be referred to extra-chromosomal bodies 

 or substances, we may use the nouns, zeiixis 

 and exozeuxis, and corresponding adjectives 

 zygous and exozygous. These alternatives 

 correspond closely with chromosomal and 

 cytoplasmic inheritance ; but " exozeuxis " 

 has an advantage over " cytoplasmic heredity," 

 since some exozygous phenomena may con- 

 ceivably be associated with nucleoplasmic 

 structures or substances instead of the cyto- 

 plasm. 



Under zeuxis or chromosomal heredity 

 three fundamental relationships of hereditary 

 factors are to be noted, depending on whether 

 only one chromosome pair or linkage group 

 is involved, or more than one, and whether 

 the chromosomes concerned are behaving in 

 typical or atypical fashion. These three cate- 

 gories may be named, respectively, monozeuxis 

 (one pair involved), pleiozeuxis (two or more 

 pairs involved), and anomozeuxis (involving 

 chromosomal irregularities), and the cor- 

 responding adjectives will be monozygous, 

 pleiozygous and anomozygous. 



The last of these categories, anomozeuxis, 

 is a composite made up of several phenomena 



