RULES AND RATIOS OF INHERITANCE 219 



or vice versa has been described as occurring In rare cases. 

 This of course changes the method of Inheritance. Details 

 need not be taken up here, since such cases are very infre- 

 quent. 



g. In some cases, as will be shown in later chapters, when 

 the chromosomes are broken into pieces by radiation, a piece 

 of an X-chromosome may become attached to an autosome, 

 remaining thus attached in later generations. In such a case 

 the genes of this piece of X, which normally give sex-linked 

 inheritance, after the breakage give autosomal inheritance. 

 Similarly a piece of an autosome may become attached to 

 X; Its genes thereafter yield sex-linked Inheritance. 



Certain Important special conditions, some of them classi- 

 fiable, as to certain features, under cases already considered, 

 are worthy of special consideration. These are the fol- 

 lowing : 



10. Both parents recessive for a single pair of genes. — 

 When two such recessive parents are mated, the nature of the 

 characters shown by the offspring differs in different cases, 

 depending on whether the recessive genes of the two parents 

 are or are not alleles; that Is, on whether they are or are not 

 at the srame locus of the same chromosome (compare figures 

 46 and 47). 



A. The recessive genes in the two parents are alleles, be- 

 ing located at the same loci of the same chromosome (figure 

 46, c). In this case no corresponding dominant gene is pres- 

 ent. All the offspring and descendants are therefore reces- 

 sive. This is the result when the two parents are alike in their 

 recessive characters, as when two eosln-eyed parents are 

 mated. It Is also the result when the characters of the two 

 parents differ, though due to different alleles of the same 

 gene ; for example, when parents with eosin eyes are mated to 

 parents with buff eyes — both of these characters being due 

 to modification of the gene at I, 1.5 in Drosophila. In such 



