APPENDIX 2 



SYMBOLS AND SYMBOLISM 



1. Cytologkal 



Genomes {Triticunt, etc.) 

 The chromosome sets of different diploid species are denoted by capital letters. 

 An allotetraploid species can thus be represented as AA BB. 



Interchanges 

 Chromosome types are usually denoted by the similarities of their pairing 

 segments. In Datura {v. Prime Type) numbers are used, as 1.2 or 2.3 (Blakeslee, 

 1932. Proc. 6th Int. Cotig. Genetics i: 104-120). In Oenothera numbers or capital 

 letters are used, as AB-BC-CD-DA for a ring of four. Interstitial segments may 

 be shown by small letters representing complexes, as AgB-BvC-CgD-DvF, etc. 

 (Darlington, 193 1. J. Genet., 24: 405-74). 



Chromosomes 

 In maize and Drosophila, chromosomes are known by Roman numerals. In 

 purely cytological descriptions chromosomes are shown either alphabetically as 

 capitals, e.g. A, B, C, D, in Crepis with index letters for the species, or by initials 

 for types as L, M and S, for long, medium, and short types in Hyacinthus. 



2. Genetical 



The convention of denoting the dominant allelomorph by a capital letter and 

 the recessive allelomorph by the corresponding small letter was introduced by 

 Mendel; but he did not attach specific letter pairs to each of his unit differences, 

 or factors, so that his symboHsm was of a general nature. This refinement, of 

 reserving each letter pair for the representation of a given gene, or factor, was, 

 however, brought into use soon after the rediscovery of Mendel's paper, and 

 the system of symbohsm thereby developed has been very widely used. It is the 

 form adopted by Bateson in all his works, and is well illustrated in his Mendel's 

 Principles of Heredity. 



The letter used to denote a given gene was generally chosen for its relation 

 to the effect of the dominant allelomorph. Thus C was used for the dominant 

 allelomorph giving colour as opposed to albinism (c) in mice, and B for the 

 dominant producing bluer colour, as opposed to red (b) in the flowers of Primula 

 sinensis. Where a number of genes were described, if all the more appropriate 

 letters had already been claimed, letters which bore no relation to the character 

 expression of the gene were used, rather than a two-letter symbol, e.g. Z-z for the 

 normal-claw leaf gene in Primula sinensis. 



In Drosophila genetics a somewhat different system has been developed. The 

 wild-type appearance of the fly is taken as the standard, and all genes are described 

 and symbolized by references to the departures which they cause from this 



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