September 9, 1921] 



SCIENCE 



215 



of diverse nature, which, have been occasion- 

 ally lumped together under the expression 

 " chromosome-exceptional," including non-dis- 

 junction (primary and secondary), triploidy, 

 tetraploidy, etc., chromosome elimination, 

 fragmentation, chromosomal fusions, rear- 

 rangements of whole chromosomes, or of genes 

 in the chromosomes, etc. 



Accepting the four categories represented by 

 the terms monozeuxis, pleiozeuxis, anomo- 

 zeuxis and exozeuxis, what is their relation 

 to Mendelism? This question can be profit- 

 ably discussed only if prefaced by a statement 

 that Mendelism is here taken to include only 

 the phenomena to which Mendel's interpre- 

 tation applies, namely to the separation of 

 each pair of alternative factors into equal 

 numbers of germ cells in both sexes, and a 

 purely chance assortment of the several alter- 

 natives among the several gametes, so that 

 the permutational groupings of unit factors 

 shall be potentially represented by equal 

 numbers of germ cells. 



Such behavior of the genes during game- 

 togenesis provides for the production of the 

 typical Mendelian ratios if there is neither 

 selective fertilization nor selective elimi- 

 nation. 



With this understanding of the phenomena 

 to which the words " Mendelism " and " Men- 

 delian " are appropriately applied, it will be 

 obvious (1) that all zygous monohybrids are 

 Mendelian. In other words, monozeuxis is 

 Mendelian if only one pair of factors is con- 

 cerned and the chromosome pair involved is 

 behaving typically. (2) Monozygous dihy- 

 brids are likewise Mendelian whenever cross- 

 ing over equals or exceeds 50 per cent. (3) 

 All pleiozygous dihybrids or polyhybrids are 

 Mendelian so long as no two factors in the 

 series are monozygous with a frequency of 

 crossing over lower than 50 per cent. (4) 

 Anomozeuxis may under certain circum- 

 stances exhibit Mendelian phenomena. Thus 

 in the case of non-disjunction, if the odd 

 (impaired) chromosome does not interfere 

 with the normal disjunction of any other pair 

 of chromosomes the genetical behavior with 

 respect to qualities determined by the un- 



paired chromosome gives the results expected 

 of a typical Mendelian monoheterozygote. 



Non-Mendelian phenomena will be found 

 (1) in monozygous dihybrids whenever cross- 

 ing over is less than 50 per cent.; (2) in 

 most cases of anomozeuxis, and (3) in all 

 cases of exozeuxis. 



In the (Enotheras where the question of 

 Mendelian or non-Mendelian heredity has 

 been most sharply and persistently raised, the 

 situation seems now in fair way to be cleared 

 up: 



1. Exozeuxis is probably concerned in tlie 

 inheritance of a variegation of the foliage 

 which is occasionally found. 



2. The hrevistylis factor which has seemed 

 thus far to be inherited independently of 

 other known factors, probably represents, in 

 relation to these other factors, a case of 

 pleiozeuxis. 



3. The occurrence of frequent irregularities 

 in chromosome behavior (anomozeuxis) is 

 illustrated (a) by the oft-repeated occurrence 

 of the 15-chromosome forms, albida, lata, 

 semilata, scintillans, hipartita, etc.; (6) the 

 triploid or " semi-gigas " individuals some- 

 times called " heroes " because of their robust, 

 gigas-Vike appearance ; (c) the tetraploid 

 gigas; and {d) by cases of probable frag- 

 mentation of chromosomes in forms with an 

 extra diminutive chromosome.^ 



4. I now have on record data which demon- 

 strate beyond question that the factors for 

 the following characters are monozygous, 

 being located in a single chromosome pair 

 (chromosome I) and at a maximum distance 

 of considerably less than 50 units: (o) 

 ruhricalyx bud pigmentation; (i) intense red- 

 dening of the stems; (c) nanella stature; (d) 

 pink-coned buds; (e) sulfurea flower-color; 

 and (/) and {g) two zygote lethals ("bal- 

 anced"). As this group of characters makes 

 up so large a block of those which have at- 

 tracted the attention of geneticists, and as 

 there are preliminary indications that still 

 other factors are linked with the factors for 



1 Lutz, Anne M., ' ' ffinothera mutants with di- 

 minutive chromosomes," Amer. Jour. Bot., 3: 

 502-526. 1916. 



