[Chap. XXXVIII HYBRID SEGREGATION 467 



Interpreting the progeny represented by the checkerboard diagrams. 

 There is no value in being able to construct checkerboard diagrams that 

 represent the progeny of a plant, or of two kinds of plants, unless one 

 can read them and visualize what they represent, after they are con- 

 structed. Since they are made to be read, one should always follow cer- 

 tain accepted conventions in making them, so that he may see almost 

 at a glance both the phenotypic and the genotypic ratios of the progeny. 



In the diagrams ( page 463 ) representing the progeny of the genotypes 

 Td and Rw it is obvious in the one that 3/4 of the plants are tall and 

 1/4 are dwarf; in the other, 3/4 of the plants are red-flowered and 1/4 

 are white-flowered. The same fact may be obtained from the diagram 

 ( page 465 ) showing dihybrid ratios where 12/16 of the plants are tall and 

 4/16 are dwarf, while 12/16 are red-flowered and 4/16 are white- 

 flowered. From this same diagram one may also learn that 9/16 of the 

 plants in the field are tall red-flowered, 3/16 are tall white-flowered, 

 3/16 are dwarf red-flowered, and 1/16 are dwarf white-flowered. All 

 these ratios refer to phenotypes, not to genotypes. It should always be 

 remembered that the checkerboard diagram represents the proportion of 

 the different kinds of plants in a field and not the actual number of plants 

 in the field, and these diagrams should be read accordingly. 



Much more can be learned by looking at the genotypic ratios, and 

 also the genotypes of particular individuals in the progeny. For instance 

 in the diagram illustrating the progeny of the genotype Td, it is obvious 

 that 1/3 of the tall plants in the garden are pure tall plants, and 2/3 

 of them are hybrid tall plants; also that all the dwarf plants are pure 

 dwarf plants. The term homozygous is often used to refer to any char- 

 acter for which the plant is a pure line, that is, contains a pair of like 

 factors, of which one factor was obtained from the sperm and the other 

 from the egg. The plants of genotypes TT and dd are homozygous with 

 respect to height, while the plants of genotype Td are all heterozygous 

 with respect to height. An absolutely pure-line plant would be homo- 

 zygous with respect to all its factors. 



In the diagram representing the progeny of the plant (TdRw) 1/4 

 of the plants are homozygous with respect to both height and flower 

 color; another 1/4 are heterozygous with respect to both height and 

 flower color. Still another 1/4 are homozygous with respect to height 

 but heterozygous with respect to flower color, while the remaining 1/4 

 are just the opposite. 



In the field one can see only four kinds of plants: tall red-flowered, 



