338 PERMANENT HYBRIDS 



of these two parts, the pairing segment and the differential segment, 

 that the different mechanisms of permanent hybrids depend. 



2. COMPLEX HETEROZYGOTES : CENOTHERA 

 (i) Inheritance in (Enothera. A complex heterozygote is a diploid 

 organism which produces two types of gametes differing profoundly 

 in genetic properties and incapable of giving homozygous offspring. 

 It breeds true primarily by virtue of the elimination of the 

 homozygous embryos. 



The determination of these properties by Renner depended on 

 his genetic analysis of a group of (Enothera species. The notion of 

 the complex heterozygote, however, is important not only in 

 relation to the special genetic properties of these species and of other 

 species and hybrids which aie now being found to resemble them, 

 but also in relation to their analogy with other permanent hybrids 

 — sex heterozygotes and allopolyploids — and to the general problems 

 of variation and hybridity. 



Inheritance and chromosome behaviour are of such a special 

 type in Ginothera and so closely related that, in order to show how 

 they differ from the normal pattern, we must consider them together. 

 The complex-heterozygote Oenothera species probably share the 

 same hereditary properties with Hypericum punctatum and Rhceo 

 discolor with which they also share the property of having permanent 

 multiple rings (Ch. V). The following hereditary properties have 

 been determined in Oenothera : — 



1. They breed true in the main when self- fertilised, but throw a 

 certain proportion (i to 2 per cent, as a rule) of divergent forms. 

 These were the mutants of De Vries. Most of the mutant forms 

 have since been found to be trisomic or tetraploid (cf. Ch. Ill), 

 and differ primarily from the same kinds of mutants in other 

 species only in the high frequency with which they appear. Others, 

 however, are diploid, and do not revert to the parental form in 

 later generations. 



2. They have a variable proportion (usually 30 to 60 per cent.) of 

 bad seeds in which the embryo has developed little or not at all 

 (Renner, 1916," cf. 1929). 



3. When crossed together they usually produce more than one 



