Knight — 82 — Dictionary 



Interzonal Fibres. — The spindle fibres which link the daugh- 

 ter chromosomes at anaphase and at the beginning of telo- 

 phase. 



Intra-genic Changes. — Mutational alterations in the nature 

 of the individual gene, as opposed to inter-genic changes, q.v. 



Intra-nuclear Mitosis. — Mitotic nuclear division which takes 

 place within the nuclear membrane without any cooperation 

 from the cytoplasm. 



Intra-sexual Selection. — Competition as between the members 

 of one sex, particularly competition as between males in re- 

 production, 



Intraspecific Selection. — Selection which operates as be- 

 tween individuals within a species, as distinct from inter- 

 specific selection, q.v. 



Introgression, — Infiltration of the germplasm of one species 

 into another; typically such infiltration would be brought 

 about by chance backcrossing. 



Introgressive Hybridization. — See Introgression. 



Inversion. — The reversal of one segment of a chromosome 

 relative to the remainder and the consequent linear reversal of 

 the gene sequence in the inverted segment relative to the rest 

 of the chromosome. 



Inversion, Overlapping. — A compound inversion caused by 

 a second inversion which includes part of a previously in- 

 verted segment. 



Inversion Heterozygote. — A zygote in whose cells one or 

 more of the chromosomes possess an inverted segment which 

 is not inverted in the partners of these chromosomes ; inversion 

 hybrid; a form of structural hybrid. 



Inversion Hybrid. — An inversion heterozygote, q.v. 



I.Q. — Intelligence Quotient, q.v. 



Irregular Dominance. — A form of dominance in which the 

 gene shows variable expression in the heterozygous state, being 

 on some genetic backgrounds coupled with certain environ- 

 mental conditions, recessive, but showing, on other backgrounds, 

 various degrees of dominance. 



Irreversibility, Dollo's Law of. — See under DoUo. 



Isauxesis. — Ontogenetic heterauxesis in which the growth of 

 the part is at the same rate as that of the body as a whole. 



Iso-alleles. — Alleles which are indistinguishable except by 

 special tests. 



Iso-allely. — The state or condition of having isoalleles, q.v. 



Isochromatid Breaks. — 1-hit chromatid breaks ; sister-re- 

 unions; aberrations which "involve breaks in both sister 



