MUTATION 



257 



normal allelomorph. They never produce a tumor and 

 never transmit the disease. The same mother that gave 

 these two kinds of sons — having been fertilized by a nor- 

 mal male, since no affected males exist— produces also two 

 kinds of daughters, one containing the gene for the tumor 

 (and its normal allelomorph), the other having two nor- 

 mal genes. The former transmit the disease as just 

 explained, the latter daughters are perfectly normal and 

 do not transmit the disease. 



Other lethal genes kill the pupae, a few of them even 

 allow the fly occasionally to come through, but such flies 

 rarely propagate. Certain races of Drosophila have ster- 

 ile or nearly sterile females, other races sterile males. 

 The sterility is here lethal in so far as it affects the germ- 

 cells. Some effects on other characters are also generally 

 to be seen. 



The presence of a lethal gene near to, i.e., linked to, 

 another mutant gene may affect the kinds of individ- 

 uals that appear because owing to the linkage the other 

 mutant character fails to appear, except when crossing 

 over takes place. Some examples of this relation may be 

 given. There is a mutant race called beaded (Fig. 109) 

 in which the margin of the wing is irregularly broken, 

 giving the appearance of a beaded edge. The gene for 

 beaded is dominant, and lethal when homozygous. 



As in the case of the yellow mouse, only the hybrid 

 (heterozygous) combination exists, and consequently 

 when two beaded flies mate they produce two beaded to 

 on^ normal fly, as shown in Fig. 110. Here the first pair 

 of vertical lines stand for the pair of third chromosomes 

 present in the egg before its reduction. The two genes 

 here involved, that for beaded and its allelomorph for 

 normal, are indicated at the lower end of the vertical lines. 

 The two corresponding chromosomes in the male are 

 represented to the right of the last. After the ripening of 

 the germ-cells each egg and each sperm carries one or 

 the other of these chromosomes. Chance meetings of egg 

 17 



