GENETIC SYSTEM: RELATION TO CHARACTERISTICS II5 



difference between the two parents in the effects of another 

 pair of autosomes. We may therefore represent the two 

 pairs of autosomes in the two parents as in figure 29. 

 Representing the dominant characters by capitals, the re- 

 cessives by lower case letters, we may in the diagram repre- 

 sent the chromosomes giving origin to the four characters 

 as follows : 



A = Yellow a = green 



B = Round b = wrinkled 



The parents will then be represented as shown In figure 

 29, The germ cells from these parents contain of course 

 one member of each pair, as shown in figure 29. One parent 

 produces gametes all of the constitution AB, the other 

 gametes all of the constitution ab. When a gametes AB 

 from one parent unites with a gametes ab from the other, 

 all the offspring (Fi) thus produced have the constitution 

 AaBb, as shown at Fi in figure 29. Since the dominant 

 autosomes A and B are both present, all these individuals 

 are dominant for both pairs of characters: that Is, they 

 all have seeds that are yellow and round, as Mendel found. 

 But all are heterozygotes for both of these characters. 



Next we mate together two of these Fi Individuals to 

 give the F2 generation. In forming gametes the rule is, of 

 course, that each gamete contains one member of each pair 

 of chromosomes. But the two pairs, Aa and Bb, are dis- 

 tributed to the gametes quite independently. Thus, some 

 get A and B, some A and b, some a and B, some a and b, 

 as shown in figure 29. 



There are thus from each parent four types of gametes, 

 AB, Ab, aB and ab, each type being represented by many 

 gametes. The gametes of each of the four types from one 

 parent unite in approximately equal proportions with gam- 

 etes of each of the four types from the other parent. That 

 is, gametes of the type AB unite in different cases with 



