APPENDIX 2 



Instead + is used as a general symbol for the wild type. Thus y cv ctj -\- implies 

 a fly heterozygous for all three genes, yellow (body colour), cross-veinlcss (wings) 

 and cut (wings), the single stroke indicating linkage of the genes. Where it is 

 necessary to specify the wild-type allelomorph of a particular gene the symbol 

 for that gene is combined with +. as-f*' or f+ in the case of the wild-type 

 allelomorph of vermihon (eye colour). This enables two different genes to be 

 designated by one letter in different cases. For example fc= black (body colour — 

 a recessive variant), while B= Bar (eye shape — a dominant variant). The 

 Drosophila system is also used for maize and some other species. Its elastic and 

 comprehensive nature is well illustrated in the introduction to Drosophila 

 Information Service, 9. 



In both systems multiple allelomorphs are indicated by superscripts to the 

 basic symbol. Some members of the set may have been named before their 

 multiple allelomorphism was discovered. This superscript then often reflects the 

 expression of the allelomorph involved, e.g. in Drosophila w is white (eye colour), 

 w' is eosin (eye colour), u/' is apricot (eye colour), u^'"°is coral (eye colour) and 

 so on. In other examples a mere stroke is used as in A'^ for Alexandra-eye in 

 Primula sinensis, or a symbol indicating the date of discovery or the name of the 

 discoverer as y-"*^ (the allelomorph of yellow discovered in April 1931) or B^ (the 

 Bar allelomorph discovered by Rapoport) in Drosophila melanogaster. 



With the Drosophila system the key symbol retains the case appropriate to the 

 first allelomorph for which is was used, but the superscript can be used to indicate 

 whether the allelomorph in question is dominant or recessive to wild type. In 

 the other system the use of the letter pairs to distinguish the dominant and 

 recessive allelomorphs breaks down with a multiple allelomorph series, especially 

 where a number of the allelomorphs are incompletely dominant in various 

 combinations. It is then customary to denote those allelomorphs which arc 

 dominant, or partially dominant, to the dominant member of the pair first 

 described, by the capital letter with a suitable superscript, the rest taking the small 

 letter with appropriate superscripts. 



One special case of multiple allelomorph symbolism which departs from the 

 above practice is that of the series controlling incompatibility in various plants. 

 S-s is generally used to denote the incompatibihty gene where it is associated 

 with distyly (when only two allelomorphs are involved) ; but with homomorphic 

 incompatibility as encountered in Nicotiatta, Veronica, etc., the symbols 5j, S2, 

 S3 . . ., etc (with subscripts) are generally used. A compatibihty allelomorph is 

 then Sy. 



Non-allclomorphic genes of similar effect are generally given names indicating 

 their similarity and symbolized accordingly. A particular example of this is 

 Lawrence's use of L and E for two complementary dominant lethal genes. Such 

 genes may alternatively be given the same name with a distinguishing number 

 which is also used in the symbol, either as an equal part or as a subscript. This 

 is especially the case for lethals and Minutes in Drosophila. 



Genes or chromosomes of related species, known or suspected of being homo- 

 logous, are generally given the same symbol with, where necessary, a distinguishing 



430 



