Duplicate genes for capsule-form in Bursa bursa-pastoris. 137 



in duplicate in every chromosome. If this be true, such a repetition 

 of genes producing independently the same character is almost certainly 

 a primitive condition, and the evolutionary progress from such a condi- 

 tion to that in which the duplicately represented character finally has 

 no representative in any chromosome and therefore disappears from the 

 soma, is plausibly traceabki through a continuous series in which the 

 duplicate genes have disappeared from one chromosome pair after another. 

 Such a descending series must be due in part to retrogressive mutation. 

 — to the loss of one or more factors, — but may be due also in part 

 to rearrangement of determiners *in some such manner as described 

 below in connection with derivative duplication. 



Emerson and East (1913) point out, on the other hand, that "if 

 in any line of descent a factor X should become located in different 

 chromosomes, or in any other way be so affected as not to be allelo- 

 morphic to itself in all combinations," a duplication of determiners will 

 result. In tliis case the dujjlication would be a derivative condition, 

 the dihybrid being derived from the monohybrid, the trihvbrid from the 

 dihybrid, and so on. A derivative duplication of determiners might also 

 result from repeated progressive mutations. For example, we may think 

 of the appearance of a new character as a result of some chemical 

 transformation, such as isomerization or polymerization, which takes 

 place in a chromosome; the nature of such a chemical change must be 

 determined by the antecedent chemical constitution of this chromosome; 

 then if we assume with Emerson that the several chromosomes may have 

 fundamentally similar constitutions, it is not illogical to l)elieve that the 

 same sort of transformation may occur independently in different chro- 

 mosomes. 



All of these methods may have been operative in different cases 

 in producing duplication of determiners, but the assumption of a physical 

 rearrangement of elements already existing appears to me to offer the 

 fewest difficulties and therefore to have been in all probability one of 

 the most potent sources of duplicate determiners. An important (juestion 

 to be considered then is how a factor may come to he located in different 

 (i. e. non-homologous) chromosomes. 



There are at least two very simple ways in which such a result 

 could conceivably come about. The normal Mendelian behavior may be 

 supposed to result from the invariable meeting of homologous chromo- 

 somes at the time of fertilization and their invariable separation into 

 different germ-cells during gametogeuesis, but the invariability of neither 

 of these processes is a logical necessity. If two chromosomes should 



Induktive Abstammungs- und Vererbungslehre. SU. \Q 



