106 THEORY OF EVOLUTION 



this peculiar result is quite simple. Every notch 

 winged female has one X chromosome that 

 carries the factor for notch and one X chromo- 

 some that is "normal". Daughters receiving 

 the former chromosomes are notched because 

 the factor for notch is dominant, but they are 

 not killed since the lethal effect of the notch 

 factor is recessive to the normal allelomorph 

 carried by the other chromosome that the 

 daughters get from their father. This 

 normal factor is recessive for notch but domi- 

 nant for life. This same figure (b) is used 

 here to show three other sex linked characters. 

 The spines on the thorax are twisted or kinky, 

 which is due to a factor called "forked". The 

 effect is best seen on the thorax, but all spines 

 on the body are similarly modified; even the 

 minute hairs are also affected. Ruby eye color 

 might be here represented — if the eyes in the 

 figure were colored. The lighter color of the 

 body and antennae is intended to indicate that 

 the character tan is also present. The light 

 color of the antennae is the most certain way of 

 identifying tan. The tan flies are interesting 

 because they have lost the positive heliotropism 



