102 GENETICS AND EUGENICS 



factor we may use C, and for its recessive allelomorph (found 

 in albinos) c, etc. 



Though a gamete, from its simplex nature, may never 

 contain more than a single allelomorph, and a zygote, from 

 its duplex origin, may never contain more than two allelo- 

 morphs, the same race may contain three or more variations 

 which belong in the same allelomorphic series; i. e., which 

 are allelomorphs of each other. In such a race, a gamete may 

 transmit any one of the series, and. a zygote may contain 

 any two, but never more. In such cases the original termi- 

 nology of Mendel, which involved the use of capitals and 

 small letters, becomes inadequate, and it has been deemed 

 advisable to use in its stead a numerical or descriptive sub- 

 script. Thus four allelomorphic conditions of the color factor 

 found among guinea-pigs have been designated C, Cd, Cr, 

 and Ca respectively. 



In calculating the result to be expected from a particular 

 cross it is obviously necessary to consider, not the number of 

 characters which the parents possess, but only the number in 

 which they differ, since as regards these only will heterozy- 

 gotes be formed in Fi, to be followed by the production of 

 new homozygous combinations in F2. Our inheritance 

 formulae therefore will contain only differential factors but 

 the student must not fall into the error of supposing these to 

 be the only factors concerned. A thousand factors held in 

 common by the parents are doubtless involved to every one 

 in which the parents are observed to differ. But factors held 

 in common are incapable of demonstration by the method of 

 experimental breeding. A factor reveals itseK only by its 

 disappearance or alteration in gametes produced by one of 

 the parents crossed. 



Both from Mendelian theory and from the experience of 

 practical breeders, it is clear that individuals which look 

 alike often do not breed alike. Hence it is useful to recognize 

 (with Johannsen) a " phenotype " as including all individuals 

 which look or seem alike, and in counter distinction to this 



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