294 FOUNDATIONS OF BIOLOGY 



brings us again to the point at which we digressed to consider 

 sex the discussion of genes associated on the same chromo- 

 some. One example must suffice to bring out the main 

 facts. 



The common form of color-blindness known as Daltonism, 

 in which the affected individual is unable to distinguish red 

 from green, has long been known to be inheritable, but in a 



XO XX 



d $ 



XO 



cf 



9 

 d 



<&> XX XX XO XO 



d 9 $ cT d 



FIG. 149. Diagram to show the inheritance of color-blindness from the male. 

 A color-blind male (shown in black) transmits the character to half of his grandsons. 

 & indicates the 'sex*' chromosome with the gene for color-blindness. (After Morgan.) 



peculiar crisscross way. The condition is transmitted from 

 a color-blind man through his daughters, who are normal, 

 to half of his grandsons; and from a color-blind woman to all 

 of her sons and none of her daughters. This behavior is 

 readily accounted for if we assume that the gene for color- 

 blindness is associated, when present, with the gene for sex 

 on the X chromosome, and that color-blindness develops in 

 males, just as 'maleness/ when it is simplex or from one 

 parent, and develops in females when it is duplex, or from 

 both parents. (Figs. 149, 150.) 



