THE MENDELIAN METHOD 



TABLE 2 



COMPOSITION OF Fj's IN PEAS (MENDEL) 



Thus the Fg consisted of three types: pure dominant (Hkc one 

 parent), mixed dominant (Hke the Fj again), pure recessive (Hke 

 the other parent), and they occurred in the proportions of I : 2 : i 

 or, expressed as fractions of the whole, j : |^ : j. 



The Segregation of Factors 



How was Mendel, and how are we, to understand these simple 

 rules and ratios ? In respect of each character the plants of the Fo are 

 all either like one or other of the two parents, or like the Fj, which 

 is of course itself merely the product of fusion of gametes from those 

 two parents. We need therefore assume only that the Fj plants are 

 producing gametes, pollen and eggs, of the types produced by both 

 of the parents. Further, since plants of the two parental types are 

 equally frequent in the Fg, gametes of the two parental types must 

 be equally frequent in the pollen and eggs of the F^. 



Take as an example the case of round and wrinkled seeds. Half 

 the pollen (and half the eggs) in F^ are of round type, R, and the 

 other half of wrinkled type, r. Then R eggs will meet R pollen 

 in ^ X ^ of the cases to give RR homozygotes (to use Bateson's term), 

 and r eggs will similarly meet r pollen in ^ of the cases to give rr 

 homozygotes. R eggs will meet r pollen in another ^ and r eggs 

 R pollen in the fourth j; these two types, being both Rr hctero- 

 zygotes, are indistinguishable and together constitute \ the cases 



(Fig- 7). 



Of course \ and i merely represents the chance of a gamete being 

 of a given kind, or of a zygote arising in a given way. The vagaries 

 of sampling will result in the exact ratios, the theoretical or expected 

 ratios, being rarely realized. 



38 



