APPENDIX I 



Dislocated Segments, homologous pairs of segments differing in their 

 linear sequence with other segments. Darlington 1937. 

 From Dislocation. Navash'm 1932. 



DisoME, Bivalent. Blakcske 1921. 



DisoMic Inheritance, arising from the determinate association of chromo- 

 somes in bivalents at meiosis. 



Dispermic, of an egg fertilized by two male gametes. 



Distal, of a part of a chromosome arm, which is further from the 

 centromere than another (Proximal) part. 



Dizygotic, from two fertilized eggs, or from the halves of one egg 

 fertihzed by two sperm, in contrast to Monozygotic (of Twins then 

 called Fraternal, etc.). 



Dominance, the relationship of two allelomorphs where the single gene 

 heterozygote resembles one of the two homozygous parents (said 

 to carry the Dominant allelomorph) rather than the other (said to 

 carry the Recessive allelomorph) on an arbitrary scale distinguishing 

 the two phenotypes. Mendel 1865. 

 Conditioned Dominance, i. Dominance affected by the presence of 



other genes. 

 2. Dominance affected by environmental variation. 

 Degree of Dominance, the extent to which the heterozygote resembles 



the homozygote for the allelomorph in question. 

 Dominance Modifier, a gene which modifies the degree of dominance 



of another gene. Fisher 1928. 

 Partial Dominance, incomplete dominance. 



Dominant Mutant, term apphed in Drosophila to any mutant whose 

 effect is detectable when heterozygous with its wild-type allelomorph. 



Double Heterozygote, heterozygote in respect of two genes. 



Double Reduction, v. Non-Disjunction. 



Drift, changes in the aggregate of genotypes in a small population resulting 

 from the random (non-selective) extinction of allelomorphs of genes, 

 in regard to wliich the population was heterogenic. Wright 193 1. 



Duplex, v. Nulliplex. 



Duplicate Genes, v. Polymeric Genes. 



Duplication, Reduphcation. 



Dyad, a pair of cells formed by a meiosis instead of a Tetrad. 



Dysgenic, opposite of Eugenic. 



EcoPHENE, the range of phenotypes produced by one genotype within the 

 limits of habitat under which it is found in nature. One genotype must 

 be taken to mean a class of closely similar genotypes. Turesson 1922. 



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