HEREDITY AND VARIATION 213 



inheritance as though it were an aggregation of independent and 

 separable characteristics, each of which is perfectly distinct and 

 may exist with any combination of other characters in a given 

 individual. These traits he called "unit characters". We now 

 know that the expression or appearance of these characters may 

 vary considerably under different conditions and that the real 

 unity lies rather in the underlying factor than in the visible (and 

 perhaps variable) character which it produces. The essential 

 point, however, is that the organism, so far as its behaviour in 

 inheritance is concerned, seems to be made up of distinct and 

 independent units. Purple flower color in peas, for example, is 

 such a unit, and may be associated with either yellow or green 

 seed color, wrinkled or smooth seed surface, tallncss or dwarfness 

 of vine, and so on. A skilful breeder may thus coml^ine and 

 rearrange the characteristics of his plants almost at will. 



Dominance. — Mendel's studies also brought out the fact that 

 when plants which are dissimilar in a given feature (such as 

 flower color, let us say) are crossed, the two characters thus 

 brought together differ markedly in ability to express themselves 

 in the resulting hybrid plant. When a pure purple-flowered 

 plant is crossed with a pure white-flowered one, for example, 

 all the offspring resemble the purple parent in their flower color. 

 Such a character as purple flower color in peas Mendel therefore 

 termed dominant and one like white flower color, which fails to 

 appear in such hybrid offspring, he called recessive (Figs. 123 and 

 126). A pair of contrasting characters like these are known as 

 allelomorphs. All the characters studied by Mendel happened to 

 show complete or almost complete dominance or recessiveness, 

 but many instances have since been found where a hybrid plant 

 resembles neither parent exactly with respect to a given charac- 

 ter-pair but is more or less intermediate between them. Such 

 cases of the incomplete or imperfect dominance of one character 

 over another in the hybrid state are much more common than 

 those in which dominance is complete. The essential fact to 

 be emphasized, however, and one that is of great practical import, 

 is that the appearance of a plant (or animal) does not necessarily 

 indicate its ancestry or its genetic make-up. Dominance, 

 partial or complete, may enable a hybrid or mongrel to masque- 

 rade as a pure or superior individual. 



Segregation. — Of much more importance than this fact of 

 dominance" in the hybrid was Mendel's discovery of the manner in 



