BASES OF CHANGH 



in chimaeras which themselves have arisen from a single original 

 mutation {cf. Fig. 22). 



In animals the effects of somatic mutation are slightly different. 

 Development and life being limited, the mixture is not a permanent 

 one. Development being less simple the mixture is also less regular. 

 As a result, the changed cells give flakes and sectors instead of layers 



Fig. 23. — Gynandromorph of Drosophila melanogastcr. The left side is XX and 

 female. One X carrying the dominant gene Notch, whose effect appears in the left 

 wing, has been lost on the right side which is therefore male and shows the effects 

 of the recessive genes of the other X, viz. ruby (eye colour), scute (bristle reduction), 

 broad (wing), and forked (bristle gnarling). The whole fly is slightly warped owing 

 to the male side being shorter than the female (from Morgan, Bridges and Sturtevant, 

 1925). 



and the product is known as a mosaic instead of a chimaera. There 

 is, however, one regular type of mixture of particular interest and 

 this is the gynandromorph. Such monsters, male in one part and 

 female in the rest, arise from a genetic difference between a pair of 

 nuclei produced at an early mitosis in the embryo. They give various 

 mixtures according to the stages and relative positions of the mitoses, 

 but perhaps the commonest is the half-sidcr of the type shown in 

 Fig. 23. They come about in a variety of ways as can be proved by 

 genetic as well as by cytological evidence (Fig. 24). 



1J2 



