Knight — 46 — Dictionary 



Double Haploid. — A diploid plant carrying a chromosome 

 set from each of two distinct species. 



Double Reduction. — The occurrence of a reductional divi- 

 sion at both (meiotic) divisions in regard to particular parts 

 of chromosomes; possible in some hybrids and polyploids; 

 hence "equational exceptions" (Darlington; Haldane). 



Double Tetraploid. — An octoploid carrying in its somatic 

 nuclei four genoms from one species and four genoms from 

 another distinct species. 



Drift. — Change in the relative frequency of two alleles in 

 a population, by chance, as distinct from such change under 

 selection. 



Duplex. — (i) Of a diploid: homozygous for a given dominant 

 gene (AA). (ii) Of a triploid: having two doses of a given 

 dominant gene and one of its recessive allele (AAa). (iii) Of 

 a tetraploid: having two doses of a given dominant gene 

 (AAaa). 



Duplicate Genes. — Identical genes but situated at different 

 loci and showing no cumulative effect. 



Duplication. — The occurrence of one segment of a chromo- 

 some twice in the same complement. 



Duplicational Polyploid. — An organism having more than 

 two homologous sets of chromosomes in its body cells derived 

 from a single parent species ; an autopolyploid, or autotetraploid. 



Dyad. — Diad, q.v. 



Dysgenesis. — Infertility, especially between hybrids which 

 are fertile with members of either parent line (Beadnell). 



Dysgenic. — Tending to impair the hereditary qualities of 

 the human race. Opp. Eugenic. 



Dysploid. — Aneuploid, q.v. 



Dysploidion. — A species composed of the morphologically 

 similar members of a dysploid series, the individuals of which 

 are sexually reproductive (Camp & Gilly). 



Dysteleology. — The supposition that nature (and especially 

 organic evolution) lacks any foreordained direction or purpose. 



Dystokia, Dystocia. — Abnormaly painful and difficult par- 

 turition. 



Dystrophy. — Perforation of the perianth by insects seeking 

 nectar so that the insects do not serve as pollinating agents. 



