C. J. Bond 123 



or it is brought about by some modifying factor introduced by the 

 gametes of the recessive female parent. 



It will be best to leave the final conclusion till all the evidence has 

 been considered. 



There are certain facts about these irregular forms of eye colour in 

 man and pied coat colour in animals which suggest that presence and 

 ahsence of gametic factors though essential are not the only conditions 

 which determine the behaviour of unit characters. 



These facts suggest that much depends on the manner in which 

 any factor is present in the gamete which carries it, and on the way in 

 which it incorporates itself with, or is incorporated by, the gamete 

 which bears the alternative fector during the process of gametic 

 union. 



We already know that much depends on the volume of the factor 

 present. 



The difference in appearance between the homo- and heterozygous 

 dominant depends on doubleness or singleness of dose of the dominant 

 character. 



In this connection I should also mention Davenport's (11) remarks 

 on the imperfection of dominance in which he concludes that alongside 

 of dominance we must place an important modifying factor, the factor 

 of the strength or potency of the representative of the given character 

 in the germ plasm. 



The influence of reciprocal gametic contribution is well known 

 in the case of sex controlled characters. 



In other cases the appearance and behaviour of unit characters have 

 been shown to depend on the intermediation of a second and in some 

 cases of a third ftictor, introduced by the gamete which carries the 

 recessive character. Thus black eye colour in certain varieties of mice 

 is attributed to the interaction of two factors (Bateson, p. 112)(9). 



The limitation of colour to certain skin areas in pied individuals, in 

 which pied pattern is dominant, has been explained by the restraining 

 influence of the factor for pied pattern on the &ctor for self colour 

 (Bateson, p. 84)(9). 



The investigation of irregular types of eye colour pattern throws 

 some light on the relationship between these different factors for eye 

 colour especially in the human subject, because in man the close 

 association which exists in the black and white races between eye colour 

 and skin colour has been partly dissolved in the European or mixed 

 races, and one complicating element has been thereby removed. 



