PHENOMENA OF INHERITANCE 335 



It is possible for one plant to have all of these dominant characters 

 or all of the recessive ones, or part of one kind and part of the other. 

 The inheritance formula of a plant having all seven of the dominant 

 characters is ABCDEFG; of one having all of the recessive characters 

 abcdefg. When two such plants are crossed the inheritance formula of 

 the hybrid is AaBbCcDdEeFfGg, and since the dominant and recessive 

 characters (or rather determiners 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 other six 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 inheritance 

 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 

 possible combinations is (128) 2 or 16,384, which represents the number 

 of combinations of these characters which are possible in the F 2 genera- 

 tion. Every one of these more than sixteen thousand genotypes may be 

 represented by various combinations 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 short hand one can show in a graphic way 

 the possible segregations and combinations of hereditary units in 

 gametes and zygotes through successive generations, and as a result 

 many modern works on Mendelian inheritance look like pages of 

 algebraic formulae. 



Some progress has been made, as was pointed out in the last lecture, 

 in identifying certain structures of the germ cells with certain hered- 

 itary units, but quite irrespective of what these units may be and where 

 they may be located it is possible, by means of the Mendelian theory of 

 segregation of units in the germ cells and of chance combinations of 

 these in fertilization to predict the number of genotypes and phenotypes 

 which may be expected as the result of a given cross. 



.4. Presence and Absence Hypothesis. — Mendel spoke of the pres- 

 ence 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 characters as positive and nega- 

 tive 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 char- 

 acterized 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 characters. 



