METHODS OF BREEDING 579 
is inversely proportional to the number of factors concerned in the 
selection, for the greater the number of factors the slower is the rate at 
which the population approaches a uniformly homozygous condition. 
Theoretically complete attainment of this condition is only reached after 
an infinite number of generations, but practically the number of genera- 
tions which is necessary to measure up to within 5 per cent. of the 
possible limit is much smaller. It is, however, often so large that the 
animal breeder would prefer to use some other method, if by so doing, 
he could more quickly reach the desired standard of excellence and 
stability of type. 
At this point, however, it should be mentioned that selection is often 
made for characters which are recessive, or which give intermediates 
when in the heterozygous condition. In such cases, of course, the 
relation between phenotype and genotype is simpler and methods of 
selection gain in effectiveness in consequence thereof. 
Phenotypic Selection Does Not Make Allowance for the Differences 
Which May Exist Among the Genotypes of a Given Phenotype.— 
Simple examples of this proposition may be quoted without number. 
In fowls for example there are dominant whites like the White Leghorn 
and recessive whites like the White Plymouth Rock. The diverse progeny 
which is obtained by mating these two breeds together has been described 
in detail in a previous section. There is some evidence that a similar 
condition may obtain in cattle with respect to white coat color. White 
is, likewise, dominant in the horse, and may therefore conceal a large 
number of latent factors. In the pig the same differences in behavior 
with respect to white coat color have been noted. There is reason to 
believe that the same kind of diversity in genetic constitution obtains 
for economic characters, as for those not so strictly utilitarian. The 
breeder who follows a method of phenotypic selection should not, therefore, 
be surprised if crossing different strains results in a disappointing lack of 
uniformity in his herd. It is not difficult to see that in differences of 
genotype such as have been noted here, the breeder of best to best meets 
one of his most perplexing problems. 
Phenotypic Sclection Fails to Allow for Heterozygosis. 56 other 
portions of this book the assumed effect of heterozygosis on vigor and 
fertility has already been discussed at considerable length. If a hetero- 
zygous condition ever can determine a more vigorous development than 
the homozygous condition, then the breeding practice of the future will 
sometimes be materially altered in order to take advantage of this fact. 
But aside from this possible difficulty there is sometimes a very real difh- 
culty in the fact that selection has set as its standard a type absolutely 
conditioned by a heterozygous genotype. The striking and ever-quoted 
instance of this fact is the Blue Andalusian fowl, which no amount of 
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