no AN INTRODUCTION TO MODERN GENETICS 



actual species which are permanent interchange heterozygotes. The 

 genus Oenothera was one of the earliest to be investigated genetically 

 on a large scale and its peculiar behaviour led de Vries^ to generaliza- 

 tions which do not apply fully to other organisms. In particular, at a 

 time when Mendel's results were only just being rediscovered, de 

 Vries attached the word mutation to a phenomenon which, as we shall 

 see, is quite different in nature to gene-mutation, although in some 

 ways resembhng it in phenotypic effect. 



The main facts of inheritance in Oenothera are that the varieties 

 breed more or less true when self-fertiHzed, but produce a fairly large 

 proportion of bad seed and also throw a certain proportion, up to 

 about 2 per cent, of unlike types which are de Vries's mutants; while on 



JL 



Fig. 52. The Formation of Metaphase Rings as a consequence of Segmental 

 Interchange. — A shows the chromosomes before the change; 6 after it; C shows 

 the pairing. If chiasmata are formed near the ends of the chromosomes, and 

 become terminalized, the four chromosomes will be associated in a ring at meta- 

 phase. The centromeres are represented by circles. 



crossing, the resulting hybrids often differ according to which species 

 is the female in the cross. The first real elucidation of this behaviour 

 came from the suggestions of Renner and MuUer.^ Renner suggested 

 that the species of Oenothera are really heterozygotes, consisting of 

 two associated complexes of genes, and forming, in the main, two 

 classes of gametes each containing one of the complexes. Muller sug- 

 gested a comparison with the balanced lethal system in Drosophila, 

 and advanced the hypothesis that neither of the complexes could 

 survive in the homozygous condition. A heterozygote AB would 

 therefore breed true, since the A A and BB zygotes would die. Renner 

 then proceeded to show that not only were lethal factors involved 

 which kill the zygote in the homozygous condition, but that gametic 

 lethal factors are also found, so that some of the species transmit only 

 one of their complexes through the pollen or through the ovules. 



The chromosomal basis for this mechanism has only recently been 

 discovered. It has been known for a long time^ that the chromosomes in 



^ de Vries 1901. 



2 Cf. Muller 1918, Renner 1925, 1929, Sturtevant 19266. ^ Gates 1908. 



