No. 530] CONCEPTION OF PUBE LINES 



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It should be carefully observed that in this scheme the 

 female genes, F or f pair when they meet (allelomorphs) ; 

 likewise the male genes pair only with male genes. In 

 fact, both genes are carried by all of the gametes. Sex- 

 ual dimorphism may appear either because one female 

 gene has become stronger than the others, or, because 

 one has become weaker. On the first view we have 

 the case where the female is heterozygous in its female 

 genes; in the latter case it is the male that is heterozy- 

 gous in its female genes. If in this latter case we as- 

 sume that the weakened female gene is contained in the 

 so-called Y-chromosome we can then understand how it 

 is that we have a degraded series of this chromosome 

 leading in some forms to its final extinction, for even its 

 disappearance leaves the formula? unaffected. On the 

 same grounds we may anticipate that in those species in 

 which the X elements are alike in the male, one X in 

 the female may be found larger than its partner, al- 

 though visible size differences in the chromosomes are 

 not essential to the scheme, since these chromosomes 

 undoubtedly contain many other factors than those of 

 sex whose presence might obscure size relations even 

 when such exist in the sex genes. 



These formulae appear more complicated than those 

 previously given, but in reality they are not so. It is 

 the presence of m in all of the gametes that gives the 

 appearance of complication. If this is omitted, as in the 

 formula given below, the formulae are no more complex 

 than those given earlier. 



Ff Female F % 



ff Male f f 



FF Female F F 



Ff Male F f 



FF Ff 



The formulae might be further simplified, if it seemed 

 desirable to do so, by simply indicating the determining 

 factor in each case as shown below; thus: 



