MENDELIAN HEREDITY 285 



and if we are concerned with ten characters, a modest portion of 

 those present in any complex animal, the result is the astonishing 

 total of 1,048,576. When we considcn* the number of unit charac- 

 ters which must be present in a human being it is easy to under- 

 stand why no two individuals are alike. 



The Back-Cross. In cases of complete dominance in multiple 

 hybrids it would be an endless task to attempt the determination 

 of genotypes by inbreeding. Self-fertilization would lighten the 

 task for the plant breeder, since he could be certain of mating 

 like with like, but the animal l^reeder would be at a loss to know 

 which of his many dominants were genotypically the same, so 

 that their crossing would be equivalent to self-fertilization. To 

 meet the difficulty of this situation it is customary to test an 

 individual by crossing it with a known recessive. Since recessive 

 characters are always apparent in the phenotype, such individuals 

 can be selected without difficulty. 



As an example let us assume that we have the F2 progeny of 

 the hybrid represented in Figure 163. We know that each of the 

 twenty-seven dominants contains the characters tall, smooth and 

 yellow, but whether homozygous or heterozygous for these charac- 

 ters must be determined. If the individual in question is mated 

 with a ttssyy , or homozy- j^^^^ ^^ ^^^^^^^ ^^^^^^ ^^ 



gous recessive, and gives dominant 



only tall smooth yellow 

 and tall smooth gi-een Gametes of -i 

 offspring we can be cer- reTesJive"^) ^^ 



tain that it was homozy- Yig. 164.— Diagram illustrating the back-cross 

 gous for l)oth of the first of a tall-smooth-yellow pea of the F2 genera- 

 two characters and heter- tion with a short-wrinkled-green homozy- 



» ,, ,1 • 1 gous recessive, 

 ozygous tor the third. 



For every heterozygous character it would give to one-half of its 

 offspring the recessive and to one-half the dominant. Those which 

 received the recessive, since they could receive only that character 

 from the homozygous recessive parent, would necessarily express 

 the recessive condition. The individual cited would therefore 

 belong to the genotype TTSSYy. The results are expressed in 

 Figure 164. 



Linkage. Although these facts justify the belief that unit 

 characters are independent of each other, it has been found that 

 some always appear tog(>ther under normal conditions. Such 



