THE GREAT RELAY RACE 



447 



uncontaminated reappearance of the character in the next generation, 

 is termed the principle of segregation. 



Thus, out of simple but perfectly controlled experiments with garden 

 peas, Mendel was able to lay down three "laws," namely, dominance, 

 independent assortment, and segregation, which together constitute the 

 essential features of what is known as "Mendelism." These funda- 

 mental laws have been confirmed many times over, in a great variety 

 of plants and animals by a host of critical investigators, and their use 

 now makes possible a precise prediction of results in experimental 

 breeding that was quite impossible before their formulation. 



Monohybrids, Dihybrids, Trihybrids, and Other Crosses 



The fundamental Mendelian laws as illustrated by a monohyhrid, 

 that is, a hybrid with respect to a single pair of characters, are com- 

 paratively simple. When two monohybrids are bred together, as 

 shown in the preceding paragraphs, the resulting progeny occur in the 

 phenotypic ratio of 3 : 1, and the genotypic ratio of 1 : 2 : 1. Dihy- 

 brids, trihybrids, tetrahybrids, etc., are increasingly comphcated, 

 but are quite understandable when it is remembered that they are 

 nothing more than combinations of monohybrids, resulting from the 

 independent assortment of the characters involved. The expecta- 

 tions for such crosses are show^l in the following table : 



As an example of the way in which a dihybrid works out, black 

 color in horses is dominant over chestnut color, and trotting gait over 

 pacing. These two pairs of characters are independent of each other, 

 so that when a black pacer is mated with a chestnut trotter, all the 

 offspring of the hybrid generation will be black trotters, since black 

 color and trotting gait are dominant characters. Then when such 

 hybrid black trotters are mated together there will be sixteen possible 



