Genie and Non-genic Parts of the Chromosome 75 



of Y heterochromatin changes some feature of chemical physiology, 

 preventing definite cells from producing their proper secretion. This 

 would mean some, though very slight, participation of heterochro- 

 matin in rather restricted chemical actions of some cells. If this 

 conclusion is correct, it involves a genetic diversity of different sections 

 of the Y heterochromatin, which, however, is probably very di£Ferent 

 from what are usually called genes. The meaning of this will be dis- 

 cussed below. (Though we are dealing here only with Drosophila, it 

 might be pointed out that in the Y of plants different sections of 

 action have been claimed by Knapp, Warmke, Westergaard. However, 

 here the Y contains parts affecting sex determination, as will be 

 discussed in detail; the relations to heterochromatin are not yet clear. ) 



Much better information comes from experiments in which the 

 amount of Y heterochromatin was not decreased but increased. 

 Mather (1941) was the first to find that the addition of an extra 

 Y-chromosome enhanced the expression of a trait assumed to be 

 under polygenic control. He concluded from this that the so-called 

 polygenes were also heterochromatic in character, a problem to which 

 we shall return. The best examples known were found for the so- 

 called podoptera mutants ( Goldschmidt, Hannah, and Piternick, 

 1951 ) . The extremest case is that of pod K, in which only males show 

 the mutant character. However, if an extra Y is introduced into the 

 female, the same phenotype is realized. In other mutants rather 

 variable effects of extra Y's were found. In some XXY combinations 

 penetrance was decreased; in the XYY male it was increased. These 

 facts are comparable to those found by Mather, who stated that 

 Y-chromosomes of different origin exert a specific, though rather 

 small, influence upon the number of chetae, some in the minus, 

 some in the plus direction. The effect is even smaller than that of 

 some environmental influences. From this Mather concluded that 

 the Y heterochromatin also contains polygenes. (We shall discuss 

 the concept of polygenes later.) A comparable fact has been found 

 by Barigozzi ( 1949, 1950& ) : the amount of Y heterochromatin affects 

 the size of wing cells and eye facets, which means an influence upon 

 cell division. Differences of effect in regard to the origin of the Y 

 were found also. This agrees well with facts and interpretations to 

 be studied below. Barigozzi is inclined to explain the results in 

 terms of polygenes, maintaining that heterochromatin is made up 

 of genes all acting on cell size and similar characters of the cell (see 

 Barigozzi and Pasquale, 1953). 



It is certain that the amount of extra heterochromatin influences 



