CHAPTER IX 



PERMANENT HYBRIDS 



Permanent Hybridity and Crossing-over — Complex Heterozygotes — 

 Inheritance and Mutation in (Enothera — Its Chromosome Basis — The 

 Differential Segments — Complex Trisomies — The Sex Heterozygote — Pairing 

 and Segregation of Sex Chromosomes — The Haplo-Diploid System — Sex 

 Determination — Evolutionary Changes. 



But in them Natures Coppie's not eterne. 



Shakespeare, Macbeth III. (2). 



I. PRIOR CONDITIONS 



Two kinds of heterozygote exist in which the difference between 

 the gametes is deeper in effect than that shown by mendehan and 

 simple structural hybrids, but in which the segregation is a simple 

 mendelian one : the difference behaves as a unit and only two 

 types of gamete are normally produced. These properties are 

 correlated with permanence ; that is to say, the heterozygous 

 condition is the property of the whole race or of a necessary part 

 of it. 



The first type is the " complex heterozygote," typically seen 

 in many (Enothera species. These species breed true to their 

 heterozygous condition owing to their producing only two kinds 

 of gamete and owing to the homozygous form of either type being 

 in viable. 



The second is that found in organisms with hereditary sex 

 determination (Ch. I). In these, where the differentiation of the 

 sexes is in the diploid phase, one sex is homozygous for the sex 

 determinants, the other heterozygous. The heterozygous sex can 

 be shown genetically to produce two kinds of gametes, one the same 

 as that of the homozygous sex, the other different ; they therefore 

 give the opposite sexes on fusing with the one kind of gamete 

 j^roduced by the homozygous sex. The heterozygous sex may be the 



335 



