128 



THE AMERICAN NATURALIST [Vol. LV 



With a sufficiently large number of characters influen- 

 cing vigor it would be impossible in practice, even without 

 the assumption of linkage, to obtain homozygous indi- 

 viduals having the vigor of the first generation. 



Thus with 10 pairs of characters over 700,000 indi- 

 viduals would have to be grown bef ore there would be an 

 even chance of obtaining an individual homozygous for 

 all of them. 



More than ten separately inherited Mendelian char- 

 acter differences affecting growth have been identified 

 and there is no reason for believing that more than a 

 small proportion have been isolated or that more than a 

 small proportion produce conspicuous morphological 

 changes that would be readily detected. 



A near approach to the vigor of the Fj might be ex- 

 pected, of course, without complete homozygosity. 



Some idea of the chances of isolating strains that are 

 practically homozygous may be obtained by calculating 

 the size of the populations that must be grown to insure 

 a reasonable chance of finding an individual homozygous 

 for say 70 per cent, of the characters. 



Table I indicates the size of the populations necessary 

 to fulfil these conditions with the number of character 

 pairs ranging from 10 to 30. 



TABLE I 



It will be seen that to have a reasonable chance of ob- 

 taining an individual homozygous for even 70 per cent, 

 of the character pairs it is necessary to limit the char- 

 acter pairs to 15 or less. Another way of gaining a 

 quantitative idea of the degree of homozygosity that 



