DOCTRINE or EVOLUTION AND RELATED IDEAS 



501 



the alternative recessive qualities will appear. This explains 

 why the inbred recessives always breed true. The recessive 

 quality cannot show at all as long as there is any taint of the 

 dominant quality. 



Fig. 2SS- 



TALL PARENT 



DWARF PARENT 



R 



THESE PARENTS FORM PURE GAMETES 

 EGGS OR SPERM 



D ^O 



PURE DWARF 

 O D 



Fig. 2SS- A diagram to show graphically the formation of gametes, dominance, possible 

 matings, segregation of qualities in the gametes, and the proportion of resulting offspring, in the 

 Pi and Fs generations of the tall and dwarf peas. Black indicates tallness; white, dwarfness. 

 Black outside the white indicates its dominance. TT, a plant in which tallness comes from each 

 parent; T(D), in which tallness comes from one parent and dwarfness from the other; DD, dwarf- 

 ness from each parent. 



Questions on the figure. — ^Why are the gametes under each original parent 

 figured alike? Why are those of the Pi parents unlike? Why are the gametes 

 always either white or black, not mixed as the parents may be? Carry for your- 

 self the figure one step further, that is to the P2 generation. What kind and pro- 

 portion of offspring would you get by crossing the possible gametes of TT X TT? 

 TT X r(D)? T{D) X T{D)f T{D) X DDf DD X DDf Make a diagram 

 of each. 



c. The principle oj segregation, or purity oj the gametes. 

 This principle asserts that, while bpth dominant and recessive 

 determiners are to be found in every primordial germ cell, both 

 determiners cannot go into one sperm or one ovum. In the 

 process of maturing ova and sperm by nuclear division the two 

 kinds of determiners separate, and thus an equal number of 

 sperm and ova. carry the dominant and the recessive characters. 



