194 THE PHYSIOLOGY OF REPRODUCTION 



in peas, the two parent varieties initially selected differing from 

 each other in one particular character. The hybrids produced 

 by crossing were all similar superficially, and resembled one of 

 the parents in the character in question, which was therefore 

 called the dominant character, the other character being known 

 as recessive. When the hybrids were crossed among themselves, 

 approximately one half of 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 Mendehan 

 conception of gametic differentiation. Breeders of blue Anda- 

 lusian 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 irre- 

 gular 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 hke the parents, a quarter were 

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



by Bateson and a large number of other workers. For a general account of the 

 Mendelian theory, and numerous references to the literature of the subject, 

 see Bateson, loc. cit. ; also Bateson, Saunders, Punnett, and Hurst, &c., in 

 Reports to the Evolution Committee of the Royal Society, Parts I., II., III., 

 IV. and v., 1602, 1905, 1606, and 1909. 



