604 



CONTINUITY OF LIFE 



paralleled the transmission of hemophilia 

 in man. From this white-eyed stock of Dro- 

 sophila, Morgan was able to show that the 

 gene for eye color was located on the X 

 chromosome and that its inheritance was 

 definitely linked with this chromosome and 

 consequently with sex. As a result of many 

 experiments, it has been found that the 

 inheritance of this sex-linked trait follows 



though they will be heterozygous, and all 

 of the males will be white-eyed because 

 their one X chromosome carries the defec- 

 tive gene (Fig. 24-13). When these Fi flies 

 are allowed to interbreed, the effect of the 

 linked condition is further demonstrated. 

 The female can produce two kinds of eggs, 

 one-half containing an X chromosome with 

 a normal gene and one-half bearing a de- 



Fig 24-14 Color blindness is sex-linked in man and is inherited the same as white eye color in 

 Drosophila. In these figures the solid black retina indicates normal vision, the broken retina 

 represents the heterozygous condition in the female, and the white retina indicates red- 

 green color blindness. 



a specific pattern (Fig. 24-13). The gene 

 for white eye color is recessive and is there- 

 fore obvious only when homozygous. Since 

 the male has but one X chromosome, white 

 eyes will appear whenever a single X chro- 

 mosome carrying a defective gene is a part 

 of the genetic make-up of the male fly. 



In a cross between a white-eyed female, 

 whose X chromosomes must each carry the 

 defect, and a normal red-eyed male, all of 

 the female offspring will he red-eyed, al- 



fective gene. The male produces two kinds 

 of sperms as usual, that is, half containing 

 an X and half a Y chromosome. The sperm 

 containing the X chromosome, however, 

 carries the defective gene (w). Random 

 fertilization will result in half of the female 

 offspring being homozygous and therefore 

 white-eyed; the other half will be red-eyed 

 but heterozygous. Half of the males will 

 be white-eyed and half red-eyed because 

 the eggs from which they came contained 



