402 D. J. SCOURF1ELD OX MENDELISM AND MICROSCOPY. 



II and \V stand for the factors determining round and wrinkled 

 peas respectively.* Owing to the number of germ-cells containing 

 each factor being equal, it also follows that each $ R and each 

 $ W will have an equal chance, other things being equal, of 

 being fertilised by either <$ R or S W, and consequently there 

 will be on the average equal numbers of the four possible com- 

 binations. This may be put in simple form as follows : — 



<J R with $ R =1 RR (i.e. pure round) ] 



3 R 9 Wi r — 3 dominant. 



^w ;; $r}= 2rw (» h y brid » ) i 



$ W ., $ W =1 WW ( ,, pure wrinkled) = 1 recessive. 



In the case previously supposed, when the hybrids are fertilised 

 bv recessives, the combination could of course only be, — 



£ W with $ R = 1 RW (i.e. hybrid round ) = 1 dominant ; 

 <£ W „ $ W = 1 WW ( ., pure wrinkled) = 1 recessive. 



Exactly the same method may be employed for two or more- 

 pairs of characters, but necessarily the statement of the com- 

 binations will be more complex. It must suffice to merely 

 indicate here the possible combinations when two pairs of char- 

 acters are joined in the hybrids. Taking R and W to represent 

 the round and wrinkled shapes, and Y and G to represent the 

 yellow and green colours of peas, the germ-cells produced will, 

 according to Mendel's law, be of four kinds, neither more nor 

 less — namely, RY, RG, WY, WG ; and as pollen grains of each 

 kind have on the average an equal chance of mating with ovules 

 of each kind, the following sixteen matings are possible and in 

 equal numbers : — 



S RY with $ RY ^ WY with $ RY 



S RY ., $ RG £ WY „ $ RG 



S RY „ ? WY S WY ., ? WY 



S RY „ ? WG S WY „ ? WG 



S RG ., ? RY ^ WG „ $ RY 



S RG „ $ RG £ WG „ ? RG 



S RG „ $ WY S WG „ $ WY 



JRC; „ ?WG rfWG » ? W( > 



If the <$ and 9. gametes bear similar factors the union is now usually 

 scribed as being homozygous; if dissimilar, as heterozygous. 



