GENETIC VARIATIONS 32 1 



mosomes to each of two of the pairs, in the 66 different 

 combinations in which two can be selected from twelve. 



The number of possible diverse types thus becomes large; 

 in Datura with its twelve pairs of chromosomes we have al- 

 ready enumerated 234 possible diverse types. It might ap- 

 pear that varieties could be multiplied practically indefi- 

 nitely in this way by adding or subtracting one or two to 

 each of three pairs of chromosomes, then to each of four, 

 and so on. But it turns out that if more than two pairs are 

 thus changed by addition or subtraction (yet not all the 

 twelve are so changed) the individual cannot live. It ap- 

 pears to be too unbalanced for development to occur. 



Yet if all the twelve chromosome pairs lack one chro- 

 mosome, or if all have one or two additional chromosomes, 

 the individuals survive and develop. There is in such cases 

 no lack of balance; all pairs are decreased or increased 

 equally. In Datura there are known types that have but 

 twelve chromosomes, one in place of each pair. These are 

 of course haploids; they live and develop. Other types are 

 known in which there are three chromosomes in place of 

 each pair, so that 36 chromosomes are present. These are 

 called triploids. Still other Daturas are known in which 

 there are four chromosomes for each of the twelve pairs, 

 making 48 in all; these are tetraploids. In all these cases, 

 the types differ from the usual diploid individuals, having 

 but two chromosomes for each of the 12 pairs, or 24 in all. 



Further, by adding or subtracting one or two chromo- 

 somes to the different groups ("pairs") in the tetraploids 

 or other types, a great number of varieties are producible 

 from a single original diploid type. Figure 66 gives dia- 

 grams of a number of those diverse types. Blakeslee and 

 his associates have produced in Datura and studied 89 

 different types of Datura produced in such ways. And these 

 are only a fraction of those that are possible. Computations 

 show that in Datura there must be 3,620 different possible 



