Knight — 42 — Dictionary 



Diheterozygote. — An organism heterozygous for two pairs 

 of genes; a dihybrid. 



Dihybrid. — A cross between parents differing in two genes, 

 or differing in two specified genes, 



Dihydroxystilbene. — A synthetic oestrogen. 



Dikaryotic Hybrid. — A 'hybrid' whose cells carry two un- 

 fused haploid nuclei, as occurs in certain fungi. 



Diluting Factor. — A minor factor which, by itself, has no 

 effect but which lessens the effect of a major factor. 



Dimegaly. — Having two types of egg, or of sperm, one larger 

 than the other. 



Dimeric. — Carrying dominant genes at both of two duplicate 

 loci. cf. Monomeric. 



Diminution. — The elimination of a portion of the chromosome 

 complement during nuclear division with the result that the 

 daughter nucleus is deficient in this part. 



Dimonoecious. — Having perfect (bisexual) flowers as well 

 as pistillate and staminate flowers on the one plant. 



Dimorphism. — (i) The occurrence of two distinct forms 

 within a species. See Polymorphism, (ii) The production of 

 two distinct types of flower on the one plant. 



Dinergatandromorph. — A 'soldier-male' type of sex mosaic 

 which occurs in ants. 



Dioecious. — Having male and female flowers on separate 

 unisexual plants. 



Dioestrum, Dioestrus. — The resting interval between heat 

 periods (oestrus) in animals. 



Di-oval Twins. — Dizygotic, fraternal, or dissimilar twins, 

 i.e. those arising from two distinct ova. 



Diphyletic. — Having two lines of descent. 



Diplo-. — The term diplo-, followed by a symbol designating 

 a particular chromosome, indicates an individual in whose 

 somatic cells both members of this particular chromosome- 

 pair are present. Diplo- is used to distinguish individuals of 

 this type from haplo- and triplo- (q.v.) individuals. 



Diplobiont. — A plant which has two flowering periods each 

 year. 



Diplobivalent. — A diplochromosome (q.v.) bivalent com- 

 prising eight chromatids. 



Diplochromosome. — A chromosome which has divided once 

 more often than it normally does in relation to its centromere 

 (cf. attached X in Drosophila melanogaster) (White; 

 Darlington). 



