344 



Quantitative Characters 



or to different environmental conditions under which the families 

 are grown, or both. In all such families fluctuating differences 

 that appear should be small. F3 families from different F2 

 plants, however, might be very different. 



The Fo from the above cross will segregate into 1 TiTi T2T2 : 

 2 TiTi T.to : 2 T^ti T2T2 : 4 Tit^ T^to : 1 T^T^ ^.^. : 2 T,ti toti : 

 1 ^1^1 T2T2 : 2 ^1^1 T2t2 : 1 ^1^1 ^2^2. If various Fo plants are 



20 r- 



FiG. 96. Histograms showing the distribution in the F2 of plants of 

 different heights from crosses involving four {a) and six (6) polygenes. 

 The Fi's would be respectively Tih Toto XX and Tih Toto T2t2 XX, as 

 explained in the text. 



selfed, the F3 families may be quite different because of these 

 differences in the genotypes. For example, the 54-cm F2 plant 

 should breed true because it is homozygous for both loci for tall- 

 ness. The 42-cm plants, however, may be TiTi ^0^2? ^1^1 T2T2, 

 or Titi Toti- The first two plants, when selfed, will produce F3 

 families, all the plants of which are 42 cm tall, because these 

 plants are both homozygous, but the T^t^ ^2^2 Fo plant, although 

 it is of the same height, will produce an F3 family consisting of 

 plants varying from 54 to 30 cm and segregating into exactly the 

 same ratio as is found in the F2. Different F3 families from dif- 

 ferent F2 plants may differ considerably in the amount of vari- 

 ability that they display, and two F3 families may be quite dif- 

 ferent even though they come from F2 plants that are phenotypi- 

 cally alike. 



