No. 6.] 



LAU' OF DICHOTOMY AV HYBRIDS. 



309 



between a species of pea with a green (g) germ and one 

 having a yellow (y} germ. Yellow is dominating. 



Gen. i. 



Gen. 2. 



Gen. 3. 



(hybrid) peas; produced 12 plants; 

 these bore : 



775,1' (hybrid + y) peas (= 75- 8 %)- 

 21 plants were produced. 



7 (33 P ure - 

 blooded r, 



because tl ey 

 bore: 



292 y peas. 



4 (66%) hybrids, 

 because they 

 bore : 



2 47..V (pure-blooded) 

 peas ( = 24.2%). 



20 plants bore: 



A 



462 y 149 g 670 green peas, 



(hybrid + y) (pure-blooded) 

 peas (= 76.4%). peas (= 23.6%). 



It is clear that if this process of crossing of the hybrids 

 continues, the propo rtion of hybrids to the whole population will 

 diminish ; for the share of pure-blooded forms breeds true ; 

 while the originally equal share of hybrids is repeatedly halved. 



If the hybrid is crossed with one of the parents instead of 

 with another hybrid, we will 



get (d + r) d = d, d + d, r, and 

 (d + ;-) r -- ef, r + ;-, r. 



In the first case all of the progeny will appear of the dominant 

 type. In the second case one-half will appear of that type. 

 This again agrees with experiment. 



In the case of dihybrids the law of alternative heritage is 

 somewhat more complicated. Imagine a lot of ripe germ cells 

 with the antagonistic qualities of any pair separated according 

 to the second principle stated at the outset. A indicates the 

 one pair of qualities and B the other ; then we shall have nine 

 classes of zygotes, the proportion of each of which is as follows : 



A. 



B. 



, d 



50% d, r 



6.25% d',d' ; I 2.5% d, r; 6.25% r, r. 



A. 25% r, r 



12.5% d, d; 25% d, r ; 1 2.5';; /-, r. 



B. 6.25% d, d; i 2.5% d, r; 6.25% r, r. 



