200 THE PHYSIOLOGY OF REPRODUCTION 



the offspring were found to be identical with their hybrid parents 

 (dominant hybrids), one-quarter resembled one of the original 

 varieties (the grandparent with the dominant character), while the 

 remaining quarter were like the other pure variety (the grandparent 

 with the recessive character). Consequently the pure dominants 

 and the dominant hybrids resembled one another outwardly, but 

 they differed in their capacity to transmit the -characteristics in 

 question, since the pure dominants alone were capable of always 

 breeding true. The recessives also invariably bred true. Mendel 

 drew the conclusion that in the hybrid the gametes (both male and 

 female) were of two kinds, which were respectively identical with 

 the two kinds represented by the gametes of the original pure 

 varieties. The differentiation of gametes carrying different 

 characters is the essential principle in Mendel's theory, the existence 

 of dominant and recessive characters, though often observable, being 

 by no means universal. 



Another example, taken from the work of Bateson and Punnett, 

 will be sufficient to elucidate further the Mendelian conception of 

 gametic differentiation. Breeders of blue Andalusian fowls have 

 always recognised the practical impossibility of obtaining a pure 

 strain of this breed. However carefully the birds are selected they 

 invariably produce two sorts of " wasters," some being pure black, 

 and some white with irregular black marks or splashes. Bateson 

 and Punnett were the first to supply the explanation. They found 

 that, on breeding from a large number of blue Andalusian fowls, on 

 an average half of the offspring were blue like the parents, a quarter 

 were black, and a quarter were " splashed- white." They conse- 

 quently drew the conclusion that the mechanism of inheritance in 

 the Andalusian fowl is comparable to what Mendel supposed to exist 

 in his hybrid' peas. The gametes of the breed, according to this 

 hypothesis, instead of being all similar and carrying the blue 

 character (as one would suppose on Weismann's theory), are of two 

 different kinds, those of the one kind being bearers of the black 

 character, and those of the other being bearers of the splashed-white 

 character. Such gametes, uniting by chance when the fowls mate 

 together, give rise to three kinds of offspring, one black-white 

 (becoming blue, actually, like the parents), one black-black, and one 

 white-white, these appearing (on an average) in the proportion of 

 2:1:1 according to the law of probability. In this particular case 

 of Mendelian inheritance, neither of the two alternative parent 

 characters (i.e. neither black nor splashcd-white) is dominant and 

 neither is recessive. Why black-bearing gametes uniting with 

 white-bearing gametes should give rise to blue individuals the 

 Mendelian theory does not attempt to explain. 



