26 MENDELIAN SEGREGATION 



The same manner of treatment will work for more 

 than three pairs of chromosomes; the number of 

 kinds of germ cells increases in geometrical ratio. 

 In most animals and plants the number of chromo- 

 somes is higher than in Drosophila, and the number 

 of pairs of factors that may show independent assort- 

 ment is, in consequence, increased. In the snail, 

 Helix hortensis, the half number of the chromosomes 

 is given as 22; in the potato beetle 18; in man, prob- 

 ably, 24; in the mouse 20; in cotton 28; in the four- 

 o'clock 16; in the garden pea 7; in corn 20; in the 

 evening primrose 7; in the nightshade 36; in tobacco 

 24; in the tomato 12; in wheat 8. If 20 pairs of 

 chromosomes are present there will be over one 

 million possible kinds of germ cells in the FI hybrid. 

 The number of combinations that two such sets of 

 germ cells may produce through fertilization is 

 enormously greater. From this point of view we 

 can understand the absence of identical individuals 

 in such mixed types as the human race. The chance 

 of identity is still further decreased since in addition 

 there may be very large numbers of factors within 

 each chromosome. 



