MENDEL'S LAW OF HEREDITY. 



229 



If it is a dominant, several individuals may have to be bred from 

 before the pure dominant is found; but once found it will breed trice. 

 When several Mendelian characters are desired in combination, a much 

 larger number of individuals will be required to secure the pure form ; but 

 once found no "fixing" will be required; it breeds true at once. The 

 old system of "fixing " by many years' selection leaves all to chance, and, 

 in the light of present knowledge, is largely a waste of time. 



When the desired "point" of the breeder is not a simple Mendelian 

 character, but rather a compound one made up of more than the simple 

 Mendelian unit, as is evident in certain cases, the breeder must rely on 

 the assistance of the Mendelian experimenter, who will proceed to analyse 

 it by exact methods. Once the Mendelian elements are discovered, the 

 compound character will be as easily worked by the breeder as a simple 

 one. The phenomenon of dominance apparently explains the oft- observed 

 facts of characters "skipping a generation." 



The facts of dominance show that the outward appearance of an 

 individual is often no guide at all to its breeding potentialities, and that 

 a knowledge of its gametic constitution is the only guide to its heredity. 

 This can only be secured by a Mendelian analysis. Recent experiments 

 with mice, rabbits, sweet peas, and stocks show clearly, e.g., that the 

 gametic constitution of albinos (whites) is often very complex. 



When more than one pair of Mendelian characters are concerned in 

 the cross, the average result can be easily calculated, e.g. a hybrid round 

 yellow pea will give : 



(3R + IW) X (3Y + IG) = 9RY + 3RG + 3WY + IWG. 



Mendel's law shows the necessity of breeding from single individuals, 

 or, if more than one be bred from, care should be taken that each individual 

 of the batch has the same gametic constitution. 



The facts of correlation or coupling of characters should be carefully 

 watched and noted by the breeder, or his calculations may be upset. 

 Last, but not least, Mendelian experiments with plants and animals have 

 shown clearly that, as a rule, reciprocal crosses give similar results. These 

 may serve a useful purpose in exposing once more the popular delusion 

 that the " male parent gives the colour, while the female parent gives the 

 form." 



Many other questions of practical value are involved in Mendel's law 

 of inheritance, but further experiments are necessary before one can 

 venture to deal with them confidently. 



