150 PHYSICAL BASIS OF HEREDITY 



the simplest one since such suppressed division has been 

 seen and can be induced in animal eggs, is that the follow- 

 ing division might be expected to be into four parts owing 

 to the doubling of the centres. 



Gregory has described two tetraploid races of Primula 

 sinensis,^ one of which arose from ordinary plants in the 

 course of his experiments. Since known genetic factors 

 were present an opportunity was given to examine into the 

 relation between the members of the four chromosomes 

 of a set. The possibilities involved are these : Assuming 

 the parents to be AA\ and aa\ and that conjugation of 

 chromosomes takes place in twos only, then if any one 

 of the four {A A' aa') chromosomes of a set may mate with 

 any other member, there will be six possible unions, viz., 

 AA', Aa, Aa', A' a. A' a', aa'. If the two derived from 

 the same parents were the only ones that can mate, only 

 two combinations are possible, AA', aa', and if the two 

 derived from the opposite parents were the only ones that 

 mate only two (but different ones) could form, viz., Aa, 

 A' a'. The genetic expectation is somewhat different 

 for each of the three cases, since the number of different 

 kinds of gametes produced is different in each. The data 

 obtained by Gregory are not sufficient to give convincing 

 evidence in favor of any one of these possibilities, although 

 as Muller has shown by an analysis of the evidence, they 

 are more in favor of the first possibility, viz., that of ran- 

 dom assortment. Gregory, without coimnitting himself to 

 the chromosome view, follows the second possibility in his 

 analysis of the case. There is, however, nothing in the 

 chromosome theory that would support the view that 

 restricts the conjugation of homologous chromosomes 

 according to their parental origins. 



There are two other species of primose, Primula fiori- 

 hunda and P. verticillata, each with 18 chromosomes that 

 have, after crossing, produced tetraploid types. In a 



* other giant races of P. sinensis examined by Keeble and by Gregory 

 are diploid. 



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