DOCTRINE OF EVOLUTION AND RELATED IDEAS 493 



Later researches support this principle in the main, but show 

 that these unit characters do influence one another. Some- 

 times they are linked or coupled in such a way that one cannot 

 be inherited without another; sometimes they repel one another 

 so that they cannot be inherited together; and sometimes a 

 quality that appears to us as one in the body is known to be 

 made up of two or more unit characters on the germ cells. 



.Fie. 255. 



TALL PARENT DWARF PARENT 



THESE PARENTS FORM PURE GAMETES f~\ 

 EGGS OR SPERM SPERM OR\GGS 



-P 



GAM \ ETES SEGRE /GATE 



PURE TALL HYBRID TALL PURE DWARF 



T (D) DO 



FIG. 255. A diagram to show graphically the formation of gametes, dominance, possible 

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

 Pi and Pj generations of the tall and dwarf peas. Black indicates tallness; white, dwarfnesi. 

 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 dwarf ness from the other; DD, dwarf- 

 ness from each parent. 



Questions on the figure. Why are the gametes of each original parent figured 

 alike? Why are those of the FI parents unlike? Why are the gametes always 

 either white or black, not mixed as the parents may be? Carry for yourself the 

 figure one step further, that is to the P 3 generation. What kind and proportion of 

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

 T(D)? T(D) X T(D)? T(D) X DD? DD X DD? Make a diagram of each. 



b. The principle of dominance. There are determiners in the 

 germ plasm which stand for these unit characters in the body. 

 When these are mingled by the union of two pure germ cells, 

 one of these (e.g., that for tallness in the peas) will dominate the 

 other so that the latter though present in all the cells, both body 



