B. P. I.— 513. 



APPLICATION OF SOME OF THE PRINCIPLES 

 OF HEREDITY TO PLANT BREEDING. 



INTRODUCTION. 



While the discussion in these pages of principles that may be applied 

 in the improvement of crops by breeding and selection will involve 

 principles other than those discovered by Gregor Mendel, the fact 

 that Mendel's principles are somewhat complex renders it necessary 

 to state them in a general way before taking up the subject of plant 

 improvement. 



DOMINANCE AND RECESSIVENESS. 



The simplest of the principles discovered by Mendel is that which 

 is usually referred to as the ^^aw of dominance." This principle 

 should hardly be called a law, because it is in no wise general and in 

 very few cases is dominance absolute. The phenomena of dominance 

 and recessiveness may be illustrated by a few examples. 



If a red-flowered variety of the common garden pea be crossed 

 with a white-flowered variety, the progeny will have red flowers. 

 According to Mendel's original conception a cross of this kind brings 

 together two antagonistic characters. The progeny inherit the red 

 flower color from one parent and the white flower color from the 

 other. It therefore has both these characters. It happens, however, 

 that the red character predominates over the white and comes to 

 expression while the white character is not visible in the cross-bred 

 individual. Mendel suggested that a character behaving as the red 

 character does in this cross should be called a '^dominant character," 

 while one behaving as the white character in this cross should be 

 called a ^'recessive character." 



If we cross a bearded variety of wheat with a smooth variety, that 

 is, one that has no beards, the hybrids thus produced either have no 

 beards or the beards will be only slightly developed. Hence, we say 

 that smoothness is dominant to beards, at least partially, or, which 

 means the same thing, that beards are recessive to smoothness. The 

 cross between polled and horned breeds of cattle has no horns, though 

 a small proportion of such cross-bred animals may have ^'scurs," 



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