94 INBREEDING AND OUTBREEDING 



vidual selected from this population to be the progenitor 

 of the next generation would most probably come from 

 the middle classes and, therefore, would be heterozygous 

 for only about half as many factors as its parent. The 

 chance that this individual would not come from the mid- 

 dle classes between 6 and 11 would be about 1 out of 10. 

 The chance that it would be completely homozygous or 

 completely heterozygous is 1 out of 32,768. If 20, instead 

 of 15, factors were involved, the chance would be 1 out of 

 1,048,576. The selection of such completely homozygous 

 individuals would be a remarkable event. If, for instance, 

 a tobacco plant, which has 24 chromosomes as the haploid 

 number, could be obtained which was heterozygous in one 

 factor pair in each chromosome and this plant were to 

 be self-pollinated and the progeny grown, 16,777,216 

 plants would have to be produced in order to provide an 

 even chance of securing somewhere in the lot one plant 

 which was homozygous in all the twenty-four factors. 

 This number of plants would require over 2000 acres of 

 land as tobacco is grown in field culture. 



This condition by which the progenitor of each gen- 

 eration in self-fertilization tends to be half as hetero- 

 zygous as its parent holds true for any number of factors 

 and in every generation. Thus it can be seen (Table II) 

 that the progeny as a whole have an equal number of 

 heterozygous and of homozygous factor pairs in respect 

 to those characters in which the parent was heterozygous. 

 So it is that in practice the reduction in heterozygosity 

 accompanying inbreeding is greatest at first, rapidly be- 

 comes less and finally ceases for all practical purposes. 

 From at the start the degree of homozygosity, with re- 

 spect to a given number of factors, increases to 99 per 

 cent, after 7 generations of self-fertilization; after 12 







