ADJUSTMENT AND BALANCE 



to be observed, no matter how long the inbreeding may be 

 continued: something like the maximum depression has been 

 reached. 



At the same time that vigour and fertility are decreasing, so is 

 the variation between the plants of any line in each generation. This 

 decrease also virtually ceases after the first five or six generations. 

 Evidently the increasing depression is due to increasing homo- 



Fa F3 F4 



Fig. 57. — Characteristic plants to show the relative heights of two inbred lines (P) 

 of maize, of the Fj^ obtained by crossing them, and of the subsequent generations 

 up to Fj, obtained by continued sclfing. The maximum hybridity, following crossing, 

 is associated with maximum height, and the reduction in hybridity by selfmg is 

 paralleled by a reduction in height. In F7 and Fg, when homozygosis in almost 

 complete, height is reduced once more to the minimum. The increase with crossing 

 is termed heterosis, and the decrease is termed inbreeding depression (based on 

 Jones, in Sinnot and Dunn, 1939). 



zygosity and to decreasing heterozygosity. The heritable variation 

 will then decrease until full, or nearly full, homozygosity is reached 

 and only non-heritable variation remains. Other things being equal, 

 an average of only i part in 32 of the heterozygosity will remain 

 after five generations of selfing, and only i in 64 after six generations. 

 As we should expect, the vigour of the original parents is restored, 

 even exceeded, by crossing inbred lines derived from different 

 varieties, or, if from tlie same variety, separated early in the 

 inbreeding programme (Fig. 57). Though each line is homozygous, 

 or nearly so, different lines are homozygous for different allelo- 

 morphs of many genes, with consequent heterozygosity in the F/s 



236 



