jgQ Arend. L. Hagedoorn and A. C. Hagedoorn. 



seems to be a mutual repulsion between A and B, so that no AB or ab 

 gametes are ever produced. In some varieties of fowls this repulsion 

 does not seem to exist, as pure strains of blue chickens occur. 



In other cases, more genes, influencing a character, some in one, 

 some in another direction, may come into play. A good instance is 

 the following. B is a genetic factor which, in mice, makes the diffe- 

 rence between recessive yellows (bb) and mice of other colours (Bb 

 and BB) D is the factor distinguishing e. g. black mice from blue 

 ones (D) from (d). Presence or absence of G makes the difference 

 between agouti and black, or between cinnamon and chocolate. 



If we now take a dilute yellow agouti mouse, which has a 

 creamish sandy colour, and whose formula, in respect to the three 

 factors named, is bdG, and mate it to a black one (BDg), the young 

 from this mating will all be agouti, the colour of the wild mouse, 

 and as exactly intermediate in colour between the two parents, as it 

 would be possible to imagine. 



Or the case of the coherent peas. Suppose, we cross an aBCdEFg 

 pea, which has no, or practically no grains sticking together, to an 

 abcDefG one, in which all the seeds always cohere. The hybrid will 

 be aBCDEFG, and because of this constitution it will be intermediate 

 between its parents. 



It seems hardly probable, that the difference between the skin- 

 colour of a Negro and that of an European, should be due to the 

 fact, that the former has a number of genes, which each tend to 

 make the skin darker, which genes would be all lacking from the 

 European. The fact alone, that their hybrid is intermediately coloured, 

 shows, we think, that the European parent of mulattoes must possess 

 one or more factors which by their cooperation to the development 

 tend to lighten the colour of the skin. For else we should have to 

 assume, that by a coincidence, all these genes which the black parent 

 has more than the white one, are of the exceptional kind, which make 

 the individual homozygous for it different from the corresponding 

 heterozygote. If it should be true, that in man there exist genes, 

 which by their cooperation to the development tend to make the 

 skincolour lighter, then it should happen that in sime crosses, especi- 

 ally in those, in which the parents differ little in skin-colour, the 

 children resemble the lighter-coloured parent. In the book on "Croi- 

 sements ethniques" by J. A. N. Perier, several such instances are 

 given, notably cases of crosses between Arabs and Negroes, or Negroes 



