Ii8 COMMON MODES OF INHERITANCE 



" Blue eyes are due to the absence of brown pigment. If 

 there is a determiner for brown pigment in the germ-plasm, it 

 will produce such pigment in the body that arises from that 

 germ-plasm. The absence of iris pigment is proof of the absence 

 of the pigment determiner from the germ- plasm. If both parents 

 lack brown pigment, their offspring, being devoid of the deter- 

 miner for brown pigment, will all lack brown pigment. As a 

 matter of experience two parents with pure blue eyes will have 

 only blue-eyed offspring/' 



The gist of Mendelism may be stated in two general propositions. 



(1) In the fertilised ovum there come together certain paternal 

 and maternal factors or genes particularly associated with particular 

 characters, but they do not mingle ; on the contrary, in the history 

 of the germ-cells of the offspring, there is a segregation of these 

 genes, so that one member of each pair goes to one daughter-cell and 

 the other to the other daughter-cell : they separate cleanly, without 

 having influenced or contaminated one another except in rare cases. 



(2) In the segregation in question there is a free and independent 

 assortment of the genes, so that the offspring of the first cross are 

 in definite proportions as regards any particular character some 

 like the grandfather, some like the grandmother, and some like the 

 parent. 



If the two parents differ in having the qualities " colour " and 

 " whiteness " the fertilised ovum will include the factors of these 

 the hybrid offspring will be coloured, colour being dominant and 

 whiteness recessive. But in the history, probably in the matura- 

 tion, of the germ-cells of the offspring, the " colour " gene will be 

 segregated in half of the germ-cells, and the " whiteness " gene in 

 the other half. In the next generation, the offspring of the hybrids 

 (bred together or with others of similar history), there will be on 

 an average 25 per cent, with colour and pure as regards that, 

 25 per cent, with whiteness and pure as regards that, and 50 per cent, 

 "impure dominants" like the parents, with the colour expressed 

 (the dominant character), but with the whiteness latent (the recessive 

 character) . 



