HEREDITY AND Sl^X 117 



to su|)|)ly tlic explanation. They found that, on breedin*^ lioni 

 a hirge number of bhie Andahisian fowls, on an average half of 

 tlie offspring were blue like the parents, a quarter were black, 

 and a quarter were " splashed-white." They consequently drew 

 the conclusion that the mechanism of inheritance in the 

 Andalusian fowl is comparable to what Mendel sup|)Osed to exist 

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

 hypothesis, instx^ad 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 b(mig bearers of the 

 s{)lashed-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 

 ])lack-black, and one white-white, these af)pearing (on an average) 

 in the proportion of 2 : 1 : 1 , according to th(3 law of probability. 

 In this particular casc^ of Mendelian inheritance, neither of the 

 two alternative parent characters {i.e. neither black nor sf)lashed- 

 white) is dominant and neither is recessive. Why black-bearing 

 gametes uniting with white-bearing gametes should give rise to 

 blue individuals tlic M(;ndelian theory does not attempt 

 to explain. 



The determiners of the hereditary characters are called 

 factors or genes. Thus in the case just described of the Andalusian 

 fowl we speak of the gene for blackness and the gene for whiteness. 

 The cross-bred bird (the first filial or F^ generation), which is 

 blue in colour, does not carry a factor for blueness but a |)air 

 of factors — those for blackness and whiteness — and these are 

 believed to reside independently in the two separate chromosomes 

 of a pair and to become segregated out in the f)rocess of gametic 

 maturation, i.e. in the reduction division. The birds belonging 

 to the second filial (or Fg geaeration) will consist, as already 

 indicated, of one-quarter carrying the factor for whiteness, one- 

 quarter with the factor for blackness, and one-half with both 

 factors in equal numbers as in the Fi generation. This ratio 

 will be understood by referring to the following diagram in which 

 it is clear that combinations 1 and 4, both of which produce 

 blue offspring, will together compose half the total number:— 



