MENDELISM 47 



with a pure-bred short or dwarf pea (tt), the offspring 

 will be tall, but will have inherited determiners for both 

 characters. Its formula will be Tt. Of course the 

 formula for the whole plant (something no one has yet 

 tried to construct) would be excessively long and com- 

 plex, but we confine ourselves to one or a few pairs of 

 characters. We are in the position of a man who might 

 be looking at a crowd. He could not follow the move- 

 ments of all the individuals at once, but he could select 

 any one or two individuals and see exactly where they 

 went and what they did. 



10. Now it appears that while each individual has Only half of 

 two sets of determiners, he can give to his progeny only 



one of these; otherwise the number would be double passed to 

 in each generation. So the cross TT X tt (we use X spring 

 to signify crossing) gives us Tt, and can give nothing 

 else, because each parent contributes one item, and the 

 one has only T to give, the other only t. 



The Tt plant is tall, because tallness is dominant and 

 dwarfness is recessive. 



n. Suppose we take the F\ individuals, which are, as So-called 

 we have said, Tt, are cross-bred or heterozygous, and l *^ e - m 

 cross them together, thus : Tt X Tt. Each parent can inheritance 

 now give T or t, and gives either quite without choice 

 or discrimination -- as we say, "at random." Let us 

 follow the fortunes of the first Tt. Of this pair, the T 

 goes out, and is equally likely to meet T or t from the 

 other parent. Thus half such 7"s will form the com- 

 bination of TT, and the other half Tt. Now the t of 

 the first parent goes out in the same way, and is also 

 equally likely to meet T or t, and in half the cases forms 

 tT, in the other half tt. We thus get the four possible 

 combinations, all equally likely; they are TT, Tt, tT, 

 and tt. This sounds confusing, but the same result may 



