Dam. — The female parent (animal breeding). 



Darwinism. — Darwin's theory that species originate by the 

 constant selection of beneficial adaptations, as a result of the 

 effect of natural selection working on the slight variations 

 that occur. 



Darwin's Theory of Sexual Selection. — See under Selec- 

 tion, Darwin's Theory of Sexual. 



Dauermodification. — A phenomenon in which environmen- 

 tally produced modification of an organism reappears in a les- 

 sening degree for one or more subsequent generations. 



Daughter Chromosome. — A half chromosome during ana- 

 phase. See under Chromatid. 



DCR. — Double cross-overs. See under Crossing-over. 



Decidua. — (i) The mucous membrane of the uterus during 

 pregnancy and at the time when it is shed (at parturition), 

 (ii) Afterbirth. 



Deconjugation. — Abnormally early separation of the paired 

 homologous chromosomes before the end of prophase in 

 meiosis. 



Deficiency. — The loss of a segment of a chromosome from the 

 genom. 



Definitive Nucleus. — A nucleus formed in the embryo-sac 

 by the fusion of the two polar nuclei. The definitive nucleus 

 later fuses with a male nucleus to form the triploid endosperm 

 nucleus. 



Degeneration. — (i) Devolution, (ii) Deterioration of a 

 variety (usually under conditions of commercial cultivation). 



Dehydroandrosterone. — This substance appears to be a 

 metabolic product of testosterone; it occurs in urine (both male 

 and female) and has androgenic activity. 



7-Dehydrocholesterol. — Vitamin Dg. 



Delayed Dominant. — Used of a character which only appears 

 to the exclusion of its alternative allele late in ontogeny. 



Deletion. — A deficiency in which an internal segment of 

 chromosome is missing, as distinct from a terminal segment. 



