158 MENDEL'S LAWS 



In order to understand the ensuing discussion with ease the reader 

 is advised to familiarize himself with the following : (i) According 

 to the Mendelian hypothesis from a cross-bred individual, if self- 

 fertilized or mated with another impure dominant, should arise 

 pure dominants, recessives, and impure dominants in the average 

 proportion of one pure dominant and one recessive to two impure 

 dominants, this proportion being due to the circumstance that the 

 dominant and recessive units should be present in the germ-cells of 

 the cross-bred in about equal numbers. If the three kinds of 

 offspring depart much from the stated proportions, it is evident that 

 something besides pure chance is at work. The impure dominants 

 of the next and succeeding generations should behave in a similar 

 manner. (2) Since an impure dominant produces an equal number 

 of dominant and recessive gametes, therefore, if such an individual 

 of any generation be crossed with, a pure dominant (which produces 

 only dominant gametes), half the offspring, on the average, should 

 be pure, and half impure dominants. If crossed with a recessive 

 half the offspring should be impure dominants and half recessives. 

 (3) Pure extracted dominants, mated together, should produce only 

 pure dominants, extracted recessives only recessives, and extracted 

 recessives crossed with pure extracted dominants only impure 

 dominants in the first cross. (4) With respect to each separate 

 character, then, pure extracted dominants and recessives are not, 

 from the Mendelian point of view, hybrids at all ; they are believed 

 to be gametically pure, having reverted wholly to one or other of 

 the types that were crossed, the other type being quite eliminated 

 from them. They should exhibit, therefore, no tincture of the 

 alternative characters. 



270. In all points, however, the doctrine of unit segregation and 

 gametic purity needs qualification. Thus, when Mendel crossed 

 round with wrinkled peas, he obtained in the second mongrel 

 generation when the proportion of dominants (pure and impure) 

 to recessives should have been as three to one in one case forty- 

 three dominants to two recessives, and in another fourteen domi- 

 nants to fifteen recessives ; and, when he crossed yellow with green 

 peas, he got in one case twenty dominants to nineteen recessives, 

 and in another thirty-two dominants to one recessive. 1 In some 

 cases extracted recessives (presumably pure) when crossed with 

 pure recessives gave dominant offspring instead of only recessives. 

 In one such case ninety-four dominants and a single recessive 

 resulted. 2 In other cases only pure dominants producing only 



1 First Report to Evolution Committee) p. 128. . a Op. cit. y p. 79. 



