144 GENETICS 



hybrids derived from such an ancestry would in- 

 clude 25Q or (3 + 1) ^ possibilities instead of 64 or 

 (3 + 1) ^, as in the case of trihybrids. When ten 

 differing characters are combined in the parental 

 generation, there would result over a million possible 

 kinds of offspring among the hybrids of the second 

 generation, (3 + 1) ^^ = 1,048,576. 



From the foregoing it is apparent that in practical 

 breeding the only hope lies in dealing with not more 

 than one or two characters at a time. Since unit 

 characters usually behave independently of each 

 other, one may breed for a single character until it 

 is segregated out in a homozygous, that is pure, 

 condition, and then in the same way obtain a second 

 character, a third, and so on. 



Thus in a few generations of properly directed 

 crosses there can be obtained combinations of char- 

 acters united in one strain that formerly were never 

 obtained at all or were only hit upon by the merest 

 chance at long intervals. Herein lies the scientific 

 control of heredity which the trinity of Mendelian 

 principles : namely, independent unit characters, seg- 

 regation, and dominance, has placed in human hands. 



13. Summary 



Three principles are concerned in Mendel's law : 

 independent unit characters, dominance, and seg- 

 regation. 



a. Independent Unit Characters. An organism, 

 although acting together as a physiological and mor- 

 phological whole, may be regarded from the point 



