GENETIC VARIATIONS 319 



in Oenothera, the evening primrose, and in Datura stra- 

 monium, the JImson Weed.- 



Just what changes are made in the inherited character- 

 istics depends on which of the several pairs of chromosomes 

 it is that has had a chromosome added or subtracted. The 

 different chromosome pairs of course are diverse as to their 

 genes and have diverse effects on characteristics. Hence 

 adding a chromosome to one pair, or subtracting a chromo- 

 some from that pair, has very different effects from the 

 same change in another pair. Since any organism has several 

 or many pairs of chromosomes, a large number of different 

 genetic types can be produced in such ways. 



In Datura, for example, there are twelve pairs of chro- 

 mosomes. Adding a third chromosome to any one of the 

 pairs gives in each case individuals of different character- 

 istics. Hence twelve different types are producible through 

 adding a chromosome in different cases to each of the 

 twelve different pairs. Also, twelve other varieties are 

 formed by taking away, in different cases, one chromosome 

 from each of the twelve pairs. Many or all of these twenty- 

 four types have been produced and studied by Blakeslee and 

 his associates. The different types differ markedly in the 

 form and size of the fruits, leaves or flowers, or in the gen- 

 eral habit of the plant. 



In other cases two of the chromosome pairs are found 

 each to have a third chromosome added to them. Such In- 

 dividuals are known as double trisomies. For 12 chromo- 

 somes there are 66 different combinations in which one 

 chromosome can be added to each of two pairs. This yields 

 66 additional types, many of which have been identified. 

 And there are 66 other types producible by subtracting one 

 chromosome from each of two pairs, in the 66 possible 

 combinations of 2 from 12. 



When Individuals having three chromosomes in place of 

 one of its pairs form germ cells, some of these germ cells 



