CONTENTS xiii 



CHAPTER 



7. Definitions ............ 104 



8. The identification of a heterozygote . . . . 105 



9. The presence or absence hypothesis .... 106 



10. Dihybrids ............ 107 



11. The case of the trihybrid ........ 114 



12. Summary ........... .117 



13. The practical application ........ 119 



14. Conclusion ............ 120 



VI. THE PURE LINE AND SELECTION 



1. Galton's law of regression ........ 121 



2. The idea of the pure line ....... 122 



3. Johannsen's nineteen beans ........ 124 



4. The distinction between a population and a pure 



line .............. 127 



5. Cases similar to Johannsen's pure lines . . . 132 



6. Selection within the pure line ...... 133 



a. Vilmorin's wheat ......... 135 



b. Clones ............ 135 



c. Parthenogenetic progeny ...... 141 



d. Homozygous crosses ........ 142 



a. Tower's potato beetles ..... 143 



6. Drosophila bristles ....... 143 



c. Pearl's 200-egg hen ...... 145 



d. Castle's hooded rats ...... 145 



7. Conclusion ............ 146 



VII. THE FACTOR HYPOTHESIS ......... 148 



1. The hereditary unit ......... 148 



2. Different kinds of genes ........ 149 



3. Complementary genes ......... 151 



4. Supplementary genes ......... 154 



a. Castle's agouti guinea-pigs ...... 154 



b. Cuenot's spotted mice ....... 155 



c. Miss Durham's intensified mice .... 156 



d. Castle's brown-eyed yellow guinea-pigs . . 157 



e. Rabbit phenotypes ........ 159 



f. The kinds of gray rabbits ...... 162 



5. Lethal genes ....... ..... 162 



6. Modifying genes and selection .... 166 



VIII. BLENDING INHERITANCE .......... 168 



1. The relative significance of dominance and seg- 



regation ............ 168 



2. Imperfect dominance ..... .... 168 



3. Delayed dominance .......... 170 



