THE GREAT RELAY RACE 



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than unit determiners, is particularly apparent when, for example, the 

 typical dihybrid ratio of 9 : 3 : 3 : 1 becomes modified into other than 

 the usual phenotypic groups. The following ratios have been dem- 

 onstrated in various dihybrid crosses: 3:6:3:1:2:1, 9:3:4, 

 10 : 3 : 3, 12 : 3 : 1, 9 : 6 : 1, 9 : 7, 10 : 6, 13 : 3, and 15 : 1. In each of 

 these cases it is still a dihybrid, made up of two monohybrids and 

 totaling sixteen possibilities involved. 



To work out a single illustration of how the factor idea gives rise 

 to a modified phenotypic ratio, let us take Bateson's famous case of 

 sweet peas, that resulted in the 9 : 7 ratio of flower color. Bateson 

 dealt with two different strains of white-flowering sweet peas that 

 bred true to the white color as long as they were not out-crossed. 

 When the two white strains were artificially crossed with each other, 

 however, all the progeny in the first generation produced purple 

 flowers. This purple color was found to be due to the combination of 

 two factors, which may arbitrarily be designated as A and B, one of 

 which was furnished by each parental strain. Neither factor alone 

 could produce the purple color since the parents were both white. 

 When the purple hybrids in turn formed their possible kinds of 

 gametes and were crossed with each other, there resulted the custom- 

 ary sixteen combinations of a dihybrid, as shown in the checker- 

 board. AAhh (white) X aaBB (white) = AaBh (purple). Gametes 

 from AaBb = AB, Ab, aB, ah. 



Of the sixteen possibilities, the nine possessing at least one A factor 

 and one B factor produced purple flowers, while the remaining seven, 

 which did not possess both the A and B factors, were whilse. It will 

 be seen that the seven phenotypically white-flowering possibilities 

 fall into three genetically different groups, namely, 3 AAhh or 3 Aahh, 

 3 aaBB or 3 aaBh, and 1 aahh. By breaking up the seven kinds 

 of white-flowering sweet peas into the genetically different groups 

 3:3: 1, and adding them to the nine purple-flowering kinds, the 

 underlying Mendelian dihybrid ratio of 9 : 3 : 3 : 1 is restored. This 



