GENETIC SYSTEM: RELATION TO CHARACTERISTICS 109 



But as the two combinations Aa and aA contain the 

 same chromosomes, the proportions may be written 



AA -f- 2Aa + aa 



The individuals of the constitution AA and Aa contain 

 the dominant chromosome A; therefore they manifest the 

 dominant characteristic, D. The individuals aa, having only 

 the recessive chromosome a, manifest the recessive char- 

 acteristic, R. Thus In this F2 generation, so far as mani- 

 fested characters are concerned, the proportions of off- 

 spring are — 



3 Dominant to i Recessive 



Thus if dominant and recessive characteristics depend on 

 the two members of a pair of autosomes, when a dominant 

 individual AA is mated with a recessive Individual aa the 

 general result Is bound to be that the Immediate offspring 

 (Fi) all manifest the dominant characteristic, while in the 

 F2 generation (produced by mating two Individuals from 

 Fi), there will be three times as many dominant Individuals 

 as there are recessives. 



Now, this is the result that was discovered by Mendel In 

 1866; it formed the foundation for the working out of 

 Mendelian heredity. The proportion 3D to iR in the F2 

 generation is commonly known as "the Mendelian ratio" ; 

 It Is one of the most characteristic proportions In this type 

 of inheritance. Mendel discovered that there are many dif- 

 ferent characteristics that yield this 3 to i ratio. If yellow 

 and green peas are crossed, in the F2 generation there are 

 3 yellows to i green; if round peas and wrinkled peas are 

 crossed, in F2 there are 3 round to i wrinkled; and so for 

 I many other characteristics in the pea plants with which 

 Mendel worked. 



And since the time of Mendel these same ratios have 

 been found for thousands of different characteristics, in 



