30 GENETIC VARIATIONS 



then in threes instead of in twos. And when the germ cell 

 with no chromosome of this pair unites with a typical germ 

 cell, an individual is formed with only one chromosome of 

 that pair; its genes are single instead of in pairs (Figure 5, Z)). 



Many cases of this sort have been discovered, in many 

 different animals and plants. It is found that such changes in 

 the numbers of chromosomes present alter the development, 

 and so yield individuals having different characteristics from 

 the usual ones. An individual with three chromosomes in 

 place of the two of one of its pairs differs in certain respects 

 from the typical individuals; an individual with but one 

 chromosome in place of one pair differs in another way. 

 These things have been most thoroughly studied in certain 

 plants, particularly the Jimson weed, Datura,^ and the famous 

 evening primrose, Oenothera. 



Just what changes in the inherited characteristics are made 

 depends on which pair of chromosomes it is that has an 

 additional chromosome, or that lacks one of its chromosomes. 

 The genes of the different chromosomes are diverse mate- 

 rials, with different functions, different effects on characteris- 

 tics. Hence adding a chromosome to one pair has a very 

 different effect from adding it to another pair; and the same 

 is true of subtracting a chromosome from a pair. Since any 

 organism has, as a rule, several or many different pairs of 

 chromosomes, a large number of different heritable types can 

 be produced in such ways. 



An example will give an idea of the variety introduced into 

 organisms in this way. In Datura, the Jimson weed, there are 

 twelve pairs of chromosomes. Adding a third chromosome to 

 any one of the pairs gives in each case different characteristics. 

 Hence there are twelve different varieties producible through 

 adding a chromosome in different cases to the twelve different 

 pairs. Also there are twelve other varieties formed by taking 



