GENETIC SYSTEM: RELATION TO CHARACTERISTICS 1 07 



somes of a certain pair. The green seed color of another 

 variety is recessive, owing to a different constitution of the 

 autosomes of that same pair. 



Call the two autosomes of that pair in the dominant 

 yellow race by the capital letters AA, while the two auto- 

 somes that give rise to the recessive green color in the other 

 race may be called aa. The dominant parent may be desig- 

 nated D, the recessive parent R, as in figure 27. 



When these two parents produce germ cells, each germ 

 cell receives of course one chromosome for the pair. The 

 germ cells from the dominant parent D have the autosome 

 A, those from the recessive parent R have the autosome a 

 (figure 27). 



The germ cells A, from parent D, unite each with a germ 

 cell a from parent R (figure 27) giving offspring (zygotes) 

 containing both A and a. These first generation offsprings 

 are commonly designated Fi (first filial generation), as in 

 figure 27. And since the characteristic carried by the auto- 

 some A is dominant, all these Fi offspring manifest the 

 dominant characteristic D (yellow, in case yellow-seeded 

 peas are crossed with green-seeded peas). 



Next observe the result of mating together two of these 

 Individuals Aa, of the Fi generation, giving the F2 (second 

 filial generation), as in figure 28. 



Each parent Aa gives, according to the general rule, two 

 kinds of germ cells, one kind carrying the dominant auto- 

 some A, the other the recessive autosome a. Each kind of 

 germ cell from one parent unites with each kind from the 

 other. That is, half the A germ cells from one parent unite 

 with A germ cells from the other parent, half with a germ 

 cells from the other parent; and the same is true for the 

 a germ cells from the first parent. This gives in equal pro- 

 portions F2 offspring (zygotes) as follows (figure 28) : 



AA + Aa + aA -h aa 



