204 



AN INTRODUCTION TO MODERN GENETICS 



too many digits, but the heterozygotes usually had five toes on the fore- 

 foot and four on the hind. The case is particularly interesting for several 

 reasons. The substitution of a five- toed for a four-toed condition is as 

 clear-cut an example as one could wish for of a pattern effect as opposed 





Fig. 96. Comb Types in Poultry. — A single, B pea, C rose, D walnut. Pea and 

 rose are each due to a separate gene, both of which are dominant to single, which 

 is the double recessive. Walnut is the double dominant. 



(After Morgan after Bateson.) 



to a substance effect. If a pattern is the result of an interaction between 

 different forces, it is reasonable to find that the formation of an orderly 

 pattern requires the co-operation of several genes if not of the whole 

 genotypic milieu. Finally, the production of a pentadactyl limb can be | 

 regarded as a reversion or atavism, and the fact that it is only possible ' 

 by a change in the whole geneotypic miUeu may explain why evolu- 

 tionary changes are in general not reversible. I 

 Finally, there are genes which normally produce an orderly new 



