Interzonal Fibres — 83 — Isomar 



chromatids at the same locus (as far as can be seen), followed 

 by lateral fusion to produce a dicentric chromatid and an 

 acentric U-shaped fragment" (Lea & Catcheside). 



Iso-chromosome. — A chromosome with two identical arms, 

 it is thought to be derived from a telocentric chromosome by 

 the junction of sister chromatids. 



Isogamete. — One of a pair of morphologically similar gametes. 



Isogamous. — Having gametes which are undifferentiated, or 

 morphologically similar. N. Isogamy. 



Isogamy. — Union of morphologically similar gametes. Adj. 

 Isogamous. 



Iscgenetic. — Of similar origin. 



Isogenic. — (i) Homozygous, (ii) Apogamous. 5"?^ Apogamy. 



Isogenic Line. — A group, all the members of which have 

 identical hereditary make-up {e.g. clone, homozygous inbred 

 line, identical twins). 



Isogenomatic. — Of zygotic or somatic nuclei: composed of 

 similar genoms. 



Isogenous. — Of plants whose male and female gametes be- 

 have similarly in the transmission of inherited characters. N. 

 Isogeny. cf. Anisogenous. 



Isogeny. — See Isogenous. 



Isolate. — A geographically restricted breeding group, such 

 as is found in an isolated village community, within which ran- 

 dom mating occurs. A Social Isolate is a breeding group 

 restricted by class or caste. 



Isolating Mechanisms. — Cytological, genetic, anatomical, 

 physiological, or ecological causes which prevent or reduce 

 sexual activity as between two or more groups of organisms. 



Isolation. — The separation of one group from another within 

 a species so that mating between the groups is prevented. 

 Geographic — is separation due to geographical features, 

 e.g. existence of separate races on separate islands. Geographic 

 isolation is divisible into spatial — and topographical — . 

 Habitat — , or Ecological — , occurs as between groups 

 which feed on different hosts, or between parasites on different 

 hosts; territoriality is a form of habitat isolation. Seasonal 

 — is brought about by differences in flowering time or in 

 mating periods. Genetic — is due to such causes as translo- 

 cations, inversions and similar sterility barriers. 



Isolation Distances. — For distances recommended in prevent- 

 ting seed contamination by spatial isolation, see Appendix 9. 



Isomar. — A line showing the geographic distribution of races 

 with equal phenotypic manifestation; phenocontour ; isophene. 



