48 Outline of Genetics 



genotypically, that is, with the same appearance and the 

 same germinal constitution. Each of these Fi indi\dd- 

 uals will produce four kinds of gametes. The possible 

 combinations of these gametes that will occur when the 

 Fi is inbred are expressed by the checkerboard. The 

 resulting F2 involves four pheno types, as follows: nos. 

 I, 2, 3, 4, 5, 7, 9, 10, 13 are tall, red-flowered individuals; 

 6, 8, 14 are tall, white; 11, 12, 15 are dwarf, red; 16 is 

 dwarf, white. This acounts for the 9:3:3:1 ratio. 



It will be noticed that nos. 1,6, 11, and 16 are homozy- 

 gotes and therefore will breed true; but the rest are 

 heterozygotes, either for one pair of characters or for 

 both, and these will split into various types upon further 

 breeding. 



The higher polyhybrid ratios run into quite a string 

 of terms, but involve no further new principles. For 

 example, the F2 phenot^-pic ratio for the trihybrid is 

 27:9:9:9:3:3:3:1, invoKdng nine pheno types (and 2 7 

 genotypes), but it can easily be worked out by the same 

 method as was used for the dihybrid. 



Thus far we have been considering Mendel's law in 

 simple form, and have enlarged but little upon Mendel's 

 original statement. The value of the law is apparent. 

 Upon its republication in 1900, it was taken up by biolo- 

 gists, and numerous breeders set to work to test it. As a 

 consequence, data for and against it began to accumulate. 

 As might be expected, there was much apparent evidence 

 against the law, but as geneticists developed a better 

 conception of the mechanism, the contradictory evidence 

 was explained away. Almost every type of inheritance 

 has now been explained according to Mendel's law. A 

 few of the more important cases will be presentecj, 



