52 HEREDITY 



ofiEspring from each parent, since the least proportion 

 of one kind that can occur is 25 per cent. Of course 

 if one had only four offspring one might not obtain the 

 proportion shown, since the numbers are subject to 

 chance variations. But with large numbers the pro- 

 portions were found to approach very closely those 

 given. 



We can sum up the results thus. A cross between 

 tall and dwarf gives taU progeny, which however do 

 not continue to breed true -with regard to this character. 

 They produce, in fact, about 75 per cent, tall and 25 

 per cent, dwarf progeny. Of the 75 per cent, tails, 25 

 are pure, the remaining 50 are impure, and apparently 

 of the same constitution as their hybrid parents. The 

 dwarf plants of the second generation breed true. 



In this experiment two striking phenomena have been 

 brought out. The first of these is dominance — the com- 

 plete resemblance of the first hybrid generation to one 

 of the parents, the characteristic of the other parent 

 being entirely suppressed. The second is segregation, 

 i.e. the separating out in the second generation — in 

 definite proportions — of the characters combined in the 

 cross. 



When Mendel's paper was first discovered, it was sup- 

 posed by many that both dominance and segregation 

 were essential features of Mendel ism, and many writers 

 were led to speak of a " law of dominance " and a " law 

 of segregation." But we now know that dominance is 

 quite an irregular phenomenon, and that it was only by 

 accident that complete dominance occurred in all the 

 cases studied by Mendel. 



Let us next look at a typical instance in which domin- 

 ance does not occur. Such an instance is furnished by 

 the crossing of the two forms of primula, known as the 

 " Chinese " and the " star " respectively. The Chinese 

 primula has rather wavy petals, -with crenated or ragged 

 edges. The " star " form has flat petals with entire 

 margins, except for a single notch in the tip of each. 

 The cross between these two types is intermediate in 

 form and easily distinguishable from either of the pure 



