MENDELIAN TERMS 101 



made is that in which hybridization occurs, {. e., in which 

 parents of unhke character are mated with each other. This, 

 following Bateson, we may call the parental generation or P 

 generation. Subsequent generations are called filial genera- 

 tions (abbreviated F) and their numerical order is indicated 

 by a subscript, as first filial (Fi), second filial (F2), etc. 

 WTien pure races are crossed the first filial generation (Fi) 

 is usually as uniform in character as the parental races. Any 

 striking lack of uniformity in Fi may be taken as prima facie 

 evidence that one or other of the parent races is impure 

 (heterozygous for one or more characters) . It is in the F2 

 generation that recombinations are formed of the characters 

 in which the parent races differ from each other. The num- 

 bers of classes of individuals obtained in F2 and their numeri- 

 cal proportions are the significant features which indicate 

 how many Mendelizing factors distinguish the parental races 

 and what their nature is, whether dominant or recessive. 



The members of contrasted pairs of Mendelian characters 

 are known as allelomorphs, i. e., alternative forms. For ex- 

 ample, colored and albino coat are allelomorphs among 

 guinea-pigs, as also are rough and smooth, long and short. 

 The dominant allelomorph is that one which is expressed in 

 the heterozygote ; the recessive allelomorph is that one which 

 is not expressed in the heterozygote. It follows that domi- 

 nant allelomorphs are regularly expressed in Fi while recessive 

 allelomorphs are as regularly suppressed in that generation, 

 but that both of them find expression in F2, though domi- 

 nants exceed recessives in F2 as three to one. 



For the simplification of inheritance formulae, Mendelian 

 factors are commonly designated by letters of the alphabet, 

 members of the same allelomorphic pair being designated by 

 the same letter, a capital being used for the dominant allelo- 

 morph, a small letter for the recessive allelomorj^h. It will 

 assist the reader to choose letters which suggest descriptive 

 names of the characters involved. Thus for the agouti factor 

 we may use A, for its recessive allelomorph a; for the color 



