96 Heredity and Environment 



ers of characters) represented by these seven pairs of letters 

 separate in the formation of the gametes, and since each separate 

 determiner may be associated with either member of the six 

 other pairs, the number of possible combinations of these deter- 

 miners in the gametes is (2) 7 or 128. That is, in this case 128 

 kinds of germ cells may be produced, each having a different in- 

 heritance formula ; and since each of these 128 kinds of male germ 

 cells may unite with any one of the 128 kinds of female germ cells 

 the number of combinations of these characters which are pos- 

 sible in the F 2 generation is (i28) 2 or 16,384, while the number 

 of different genotypes is (3) 7 or 2187. Every one of these more 

 than two thousand genotypes may be represented by various com- 

 binations of the letters ABCDEFG and abcdefg. 



When many characters are concerned it is difficult to remember 

 what each letter stands for, and consequently it is customary in 

 such cases to designate characters by the initial letter in the name 

 of that character. By this form of shorthand one can show in a 

 graphic way the possible segregations and combinations of heredi- 

 tary units in gametes and zygotes through successive generations, 

 and as a result many modern works on Mendelian inheritance look 

 like pages of algebraic formulae. 



4. Presence and Absence Hypothesis. Mendel spoke of the 

 presence of contrasting or differentiating characters in the plants 

 which he crossed, such as round or wrinkled seeds, tall or short 

 stems, etc. Many other writers regard these contrasting charac- 

 ters as positive and negative expression of a single character, and 

 consequently they speak of the presence or absence of single 

 characters : thus round seeds are due to the presence of a factor 

 for roundness (A} while wrinkled seeds are characterized by 

 the absence of that factor (a). Round seeds are wrinkled seeds 

 plus the factor for roundness. Most of the phenomena of Men- 

 delian inheritance are more simply stated in terms of presence or 

 absence of single characters than in terms of contrasting charac- 

 ters. But it is practically certain that recessive characters are not 

 due to the absence of factors for dominant characters ; there are 



