[Chap. XXXVIII 



HYBRID SEGREGATION 



463 



The visible character that develops is referred to as the phenotijpe. For 

 example, if 400 progeny of a hybrid plant of genotype Td are found to 

 differ from each other in the proportion of 100TT:200Td:100dd, the 

 genotypic ratio is 1:2:1; but the phenotypic ratio — what one sees in the 

 field or garden — is 3:1. These ratios are often called monohybrid ratios 

 because they represent the progeny with respect to a single character. 



All the plants that have the factors TT are pure-line tall plants; the 

 eggs and sperms produced in the pollen and embryo sacs in these plants 

 each contain the factor T. If only self-fertilization occurs, each individ- 

 ual in the progeny will have the factors TT, since one height factor is 

 obtained from the egg and the other from the sperm. Similarly, the 

 sperms and eggs produced in the pure-line dwarf plants of the genotype 

 dd each contain the factor d, and all the progeny obtained by selfing 

 have the factors dd. When plants having the factors TT and dd are 

 crossed, each individual in the progeny has the factors Td, because the 

 fertilized egg from which it develops has the factor T from one gamete 

 and the factor d from the other one. 



For convenience in representing the proportions of the dififerent kinds 

 of progeny of hybrids, the checkerboard diagram is often used. If the 

 height factors in a plant are represented by Td, then half of the sperms 

 will have the factor T, the other half will have the factor d. Half of the 

 eggs also will have the factor T, the others will have the factor d. Simi- 

 larly, in a plant having the factors of flower color, Rw, half of the sperms 

 will have the factor R; the other half the factor w. Half of the eggs will 

 have the factor R; the other half will have the factor w. Now if we indi- 

 cate the factors in the eggs on one side of a checkerboard diagram and 

 the factors in the sperms along a side at right angles, we can indicate 

 quickly ( 1 ) the possible chances of union of the different kinds of sperms 

 and eggs, and (2) the proportion of the different kinds of progeny. 



R 



w 



Genotypes 

 of F2 gener- 

 ation 



R 



Genotypes 

 of F2 gener- 

 ation 



Within the checkerboard diagrams are represented the genotypes of 

 all the different kinds of progeny of each of the hybrid parents having 

 the genotypes Td and Rw, and also the ideal proportion in which they 

 may be expected to occur, especially if the total number of progeny in 



