Studies on variation and selection. ijq 



repulsion" between two genes. Anyway, in animals it is certainly 

 the rule that two individuals, genotypically identical, with the 

 exception that one is pure, the other impure for the same gene, are 

 indistinguishable. 



The second strained point in Lang's tentative explanation, is, 

 that he assumed, that in the crosses which would result in cases of 

 blending inheritance, one of the forms crossed possessed all the genes 

 in question, whereas the other form lacked them all. 



Or rather, this assumption is directly related to the preceding 

 one, as cause and effect, for it is clear, that, if one parent has some 

 genes, which all tend to influence a certain quality in one direction, 

 the other parent lacking them all, the offspring of such a couple 

 would have the quality developed to the same extent as the first 

 parent, unless heterozygotes and homozygotes differed visibly. Now, 

 we consider it exceedingly unlikely that both unprobable possibilities 

 would coincide. Neither is moreover necessary for Lang's explanation 

 of the facts. If one parent has a gene which tends to influence the 

 development in such a way, that a certain quality changes in one 

 direction, which gene the other parent lacks, whereas this other parent 

 has a gene, influencing this quality in the opposite direction, the 

 resulting offspring will be intermediate. 



To give an example. A blue Andalusian fowl, when mated by 

 us to "recessive" white hens did not produce as many blue as white 

 chicks, as should result on the hypothesis, that the white Andalusian 

 is a recessive white (blue and black Andalusians being heterozygotes 

 and homozygotes for one single genetic factor), but exclusively blacks 

 and blues in equal proportions. 



This would give us the following hypothesis of what causes the 

 peculiar inheritance of colour in the Andalusian fowl. Black Ada- 

 lusians possess a gene, absent from the whites, which gene, which 

 we will call a, together with all those genes, which all Andalusians 

 have in common, produces a black fowl. They lack another gene, 

 B, which is present in the white Andalusians (which in their turn 

 lack a). This factor b, present in a pigmented fowl, actively "dilutes" 

 the colour. It has no effect in the white Andalusians, because these, 

 as they lack a, are not pigmented. We should therefore expect dilute 

 black (blue) young from the cross black x white, which, inter se, would 

 give ab, Ab, aB and ab offspring. Now, there is no evidence that in 

 Andalusians there are ever produced aabb animals, or AABB. There 



