Disc Floret — 45 — Double F, 



Dollo's Law of Irreversibility. — Evolution is reversible 

 in that structures or functions once gained may be lost, but 

 irreversible in that structures or functions once lost can never 

 be regained (Needham). 



Dominance. — See under Dominant, Irregular , Mock 



, Conditional , Conditioned , Partial , Semi 



, Fancier's . 



Dominance, Hypothesis o£ Antithetical. — In hybrids be- 

 tween extremely diverse parents, natural selection will tend 

 to encourage those modifiers favouring one parental extreme 

 or the other and suppressing intermediates (Anderson & 

 Erickson). 



Dominant Character. — A character possessed by one parent 

 of a hybrid which appears in the F^ to the exclusion of the 

 allelic character from the other parent. By extension, the 

 gene controlling such a character is said to be dominant or to 

 show dominance. Dominance may be complete, incomplete 



(partial) or absent. Where the character is measurable the 



I- 



degree of dominance may be expressed as 100 — where h is 



d 



the deviation of the heterozygote from the mean of the two 



homozygotes and d is half the difference between the two homo- 

 zygotes. 



Dominigenes. — Modifying genes which are able to modify 

 the dominance of another gene. 



Donor Parent. — That parent from which, by backcrossing, 

 one or more genes are transferred to the backcross parent. 



Dosage Compensation. — The effect whereby a single dose 

 of a sex-linked gene has the same developmental result in 

 the heterogametic sex as a double dose has in the homogametic 

 sex. 



Dosage Indifference. — The effect whereby in certain or- 

 ganisms the male may possess several Y-chromosomes, or 

 none at all, without upsetting the genetic balance of the in- 

 dividual. 



Double Cross. — Double Fj, g.v. 



Double Diploid. — Allotetraploid, q.v. 



Double Dominants. — Two dominant, non-allelic factors 

 which together produce a certain phenotypic character which 

 does not appear in the absence of either or of both of them; 

 dominant complementary factors. 



Double Fj. — A cross between two Fj^s such that the hybrid 

 has all four grand-parents different. 



