An Introduction to a Biology 



was this : 50 of the 75 were hj^brids because 25% of their 

 offspring were " green " and 75% " yellow " (which we will 

 now write instead of " green-seeded " and " yellow-seeded "), 

 while the remaining 25 proved themselves to be pure yellows 

 by breeding true — by producing only " yellows." That is 

 to say, the 75 yellow peas are composed of 50 hybrids and 

 25 dominants ; and now that we have at last found out 

 what they are, let us look at the whole result of breeding 

 from the hybrids. We see immediately that : — 

 25% greens -f 75% yellows 



is really represented by 

 25% pure greens 50% hybrid yellows 25% pure yellows 



or 

 25% recessives 50% hybrids 25% dominants. 



Mendel experimented with the new hybrids (i.e. the 

 children of the first hybrids) and found that they, too, pro- 

 duced offspring, 25% of which were green and 75% yellow ; 

 and he fomid (though he was working with small numbers : 

 Bateson, :02, p. 57) that, for at most six generations, it was 

 a general rule that hybrids when paired together gave 25% 

 recessives, 50% hybrids, and 25% dominants. This pheno- 

 menon is spoken of as segregation ; which consists in the 

 dispatch by the hybrids, at each generation, of offspring into 

 the dominant and recessive ranks from which (so long as like 

 mates with like) there is no returning. 



If we symbolise the dominant character by Z), the reces- 

 sive by R, and the hybrid by DR (and if we imagine for the 

 sake of simplicity that each plant produces 4 seeds), we 

 see that the proportions of D's, Z)i?'s, and i^'s in successive 

 generations are as shown in the table on the following page. 

 The ratio of Z)'s, Z)i?'s, and Jf^'s can be foretold in any 

 generation n, by this formula : 



D : DR : R 

 2"— 1 : 2 : 2"— 1. 

 "In the tenth generation, for instance, 2" — 1 = 1,023. 

 There result, therefore, in each 2,048 plants which arise in 



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