468 TEXTBOOK OF BOTANY 



tall white-flowered, dwarf red-flowered, and dwarf white-flowered. But 

 from the diagram we may learn that of the tall red-flowered plants in the 

 field only 1/9 are pure tall red-flowered, 4/9 are hybrid tall red-flowered, 

 2/9 are homozygous with respect to height and heterozygous with 

 respect to flower color, and the remaining 2/9 are just the converse of 

 the preceding. Of the tall white-flowered plants, 1/3 are pure tall white- 

 flowered. One third of the dwarf red-flowered plants are homozygous 

 with respect to height and flower color. All the dwarf white-flowered 

 plants in the field are pure dwarf white-flowered. 



Ascertaining the genotype of parents. If one were told that a dwarf 

 white-flowered pea plant was crossed with a tall red-flowered one, and 

 that all the progeny were tall but some of them were white-flowered, he 

 should be able to state at once the genotypes of the two parents. 



Disregarding the progeny for a moment, we know that the genotype 

 of the dwarf white-flowered parent was ddww, and that the tall red- 

 flowered parent contained the factors T and R. Since all the progeny 

 are tall, we are sure that the other height factor in it was also T. Since 

 some of the progeny are white-flowered, we can be sure that the other 

 flower-color factor was not R, and must have been w. Hence the geno- 

 types of the two parents were ddww and TTRw. 



Inbreeding. Attention has already been called to the fact that con- 

 tinued inbreeding of hybrids and their progeny results in a number of 

 inbred lines that are homozygous with respect to many of their heredi- 

 tary factors. A hint of this fact is evident in the checkerboard diagram 

 of the progeny of the plant of genotype TdRw, since 1/4 of the plants 

 of the F2 generation are homozygous with respect to height and flower 

 color. A much better idea of the effect of inbreeding may be obtained 

 by working out a diagram for several successive generations, beginning 

 with three characters in a hybrid. Of course, the making of these dia- 

 grams becomes increasingly tedious as the number of characters repre- 

 sented increases. If all the fourteen characters discussed in this chapter 

 were considered, the number of different eggs and sperms would be 

 equal to the seventh power of 2, or 128. 



Obviously, inbreeding may be undesirable when it increases the 

 homozygosity of undesirable factors, or when it decreases the number 

 of desirable factors one wishes to have in one plant. Following inbreed- 

 ing, these desirable factors may be brought into one plant by crossing 

 inbred lines from which the undesirable factors have been eliminated. 

 On the other hand, inbred lines that are homozygous for several desired 



