Knight — 80 — Dictionary 



Induction, Somatic. — The transmission to the offspring of the 

 effects of use and disuse. 



Industrial Melanism. — The darkening of a population due 

 to the selective effect of the darkened surroundings of an in- 

 dustrial area. 



Inert Chromosome. — A chromosome which appears to have 

 few or no physiologically active genes. 



Infundibulum. — The fimbriated, funnel-shaped portion of the 

 oviduct immediately adjacent to the ovary. 



Inhibiting Factors. — Inhibitors, q.v. 



Inhibitors. — Genes which show a special form of epistasis 

 such that one gene renders another non-allelic gene entirely 

 inoperative. 



Initial Meiosis. — Meiosis which takes place in the first two 

 divisions after fertilization, at the beginning of the life cycle 

 of an organism; zygotic meiosis. 



Initial Spindle. — A miniature spindle-like structure seen in 

 the centrosome when the centriole divides; netrum. 



Innate. — In-born, inherited. 



Inositol. — A component of the vitamin Bg complex. 



Insertional Translocation. — The inclusion of a fragment of 

 one chromosome in a non-homologous chromosome in a non- 

 terminal position. 



Insertion Region. — The centromere, spindle attachment or 

 kinetochore ; a non-staining localized region in each chromo- 

 some to which the spindle 'fibre' appears to be attached at 

 metaphase. The centromere remains single for some time after 

 the rest of the chromosome has divided and at anaphase, starts 

 to move towards the pole before the rest of the chromatid. 



Instaminate. — Lacking stamens. 



Instantaneous Sympatric Speciation. — See Sympatric Spe- 

 ciation, Instantaneous. 



Intelligence Quotient. — By intelligence tests an individual 

 can be assigned to a "mental age" group (on the basis of the 

 average marks obtained by a typical group of that age). This 

 "mental age" divided by the actual age of the individual gives 

 the "mental ratio" and this multiplied by 100 is the "intelligence 

 quotient." 



Intensifying Factors, — Modifying factors which increase the 

 effect of a major gene. 



Interacting Mutations. — Mutations which give rise to com- 

 plementary factors, q.v. 



Interaction Theory. — A theory of quantitative factor in- 

 heritance which assumes that the effect of each factor on the 



