40 Mutations and Evolution. 



to be homosynaptic. After heterosynapsis the reductions will be 

 XX Y or X XY. From the former the eggs after maturation 

 will be XX and Y; from the latter, $X and XY. Also from 

 homosynapsis, the reductions will be X XY or XY X, since the 

 unmated Y may remain in the egg or enter a polar body, and the 

 eggs will therefore be X and XY. Hence 4 classes of eggs result : 

 XX and Y 4% each, of single origin ; X and XY 46% each, of 

 composite origin. Therefore if such an XXY female having 

 vermilion eyes is crossed by a wild male (red eyes), there will be 8 

 classes of zygotes, as shown in the diagram (Fig. 2). 



Of these eight classes, (2) are not exceptions but would 

 produce exceptions in the next generation, (4) are produced by the 

 reverse of the ordinary method, i.e. t by the union of a Y egg and an 

 X sperm. While themselves exceptions, they are ordinary males 

 and can neither produce exceptions nor transmit the power of 

 doing so, (6) are not exceptions, but some of their daughters will 

 get the extra Y (from XY sperm + X egg) and so produce 

 secondary exceptions. It was found that homosynapsis occurs 

 much oftener than heterosynapsis, but otherwise the chromosome 

 distribution is according to chance. That matroclinous exceptions 

 had developed from fertilised eggs was shown by the presence of 

 paternal characters derived from other chromosomes. 



In XYY males, synapsis may be of the XY or YY types. If 

 these are simply by chance there should obviously be twice as 

 many XY as YY synapses. The synapsed chromosomes disjoin 

 and the other one apparently goes equally to either pole. Hence in 

 XYY males 4 classes of sperm will be produced, X and YY from 

 heterosynapsis (small classes) and XY and Y from homosynapsis 

 (twice as large and from two sources). These four classes of sperm 

 fertilizing an ordinary female would give the following results : 

 Sperm X YY XY Y 

 Eggs X 



XX XYY XXY XY 



?<??<? 



Part the daughters will therefore be XX and part XXY, while 

 some of the sons will be XY and some XYY, again producing 

 exceptions. Primary non-disjunction in the female, or failure of the 

 sex chromosomes to separate in the egg, occurred 12 times in 

 breeding experiments involving 20484 flies, or 1 in 1700. The fact 

 that XO males are sterile shows that the Y chromosome has some 

 function in spermatogenesis, if only to serve as a balance wheel. 



