174 THE THEORY OF THE GENE 



larities are much more frequent in triplo-typic individ- 

 uals than in normals. 



From the 15 chromosome types two kinds of germ-cells 

 are expected, one with 8, one with 7 chromosomes. It has 

 been shown that these two kinds are produced. From a 

 genetic standpoint the lata type, crossed to a normal 

 type, should produce equal numbers of lata (8+7) and 

 normal (7+7) offspring. This is approximately what 

 happens. 



The most interesting question concerning these triplo- 

 types relates to the particular chromosome that becomes 

 the supernumerary. Since there are seven kinds of chro- 

 mosomes, we may anticipate that any one may appear in 

 triplicate. De Vries has recently suggested that there are 

 seven trisomic types in Oenothera, corresponding to the 

 seven possible supernumeraries. 



It is also important to bear in mind that types with two 

 supernumeraries (either like or unlike), the tetrasomic 

 types, may not be as viable as trisomic types. It is known 

 that such types occur. For instance, amongst the off- 

 spring of a triplo-type there seems to be a good chance 

 for the formation of an individual with two like super- 

 numeraries when an 8-chromosome pollen grain fertilizes 

 an 8-chromosome egg. This would give a tetra-type or 

 tetrasomic group for one particular chromosome. It 

 would be a stable type to the extent that 8 paired chro- 

 mosomes are present in each germ-cell, but it might be 

 even more unbalanced than a triplo-type with only one 

 extra chromosome. Sixteen-chromosome types have been 

 recorded, some of which are probably multiples of the 

 same chromosome when they are derived from a 15 triplo- 

 type, but their relative viability is not recorded. 



It seems, a priori, possible that duplication of any pair 

 of chromosomes may be brought about through a triplo- 

 type giving rise to a tetratypic individual. But even if 



