16 MENDELISM 



The DD and D[R] plants look alike, and can only be 

 distinguished by breeding. This being so, it follows that 

 it is necessary for the horticulturist to keep the seeds of 

 each of these plants separate if he wishes to find out 

 which particular plant is the pure dominant. Hitherto it 

 has been customary to mix the seeds of all individuals 

 which appear alike, with the result that plants grown from 

 them have shown various characters. 



How is dominance accounted for? Messrs. Bateson 

 and Punnett attribute it to the presence of some factor or 

 ingredient, and the recessive character to its absence. 

 Now, if recessives are marked by -the absence of some 

 factor, it follows that they can never reproduce the 

 dominant character which depends for its manifestation 

 upon the presence of this factor. A dwarf plant can never 

 reproduce a tall, for tallness depends upon something which 

 is absent from the dwarf. This has a most important 

 bearing upon certain inherited diseases, for if the disease 

 be dominant, owing to the presence of something, then 

 the recessive individuals will not inherit it, and will not 

 transmit it to their offspring. 



Let us now summarize the Laws that appear to me 

 to have been established as regards a single pair of 

 contrasted characters. 



(i) D x D produces DD. 



(2) R x R .. RR. 



(3) D x R 



(4) D[R] x R 



(5) D[R] x D 



(6) D[R] x D[R] 



D[RJ 



D[R] + R in equal numbers. 



D[R] + D 



3D+1R, 



or more correctly 1 DD + 2 D[R] + RR. 

 Law 4 may be shown to be true, thus — 



