66 



Since the wild males (RWX) are 

 heterozygous fcr red eyes, and the 

 female (RXRX) homozygous, it seems 

 probable that the sport arose from a 

 change in a single egg of such a sort 

 that instead of being RX (after reduc- 

 tion) the red factor dropped out, so 

 that RX became WX or simply OX. 

 If this view is correct it follows that 

 the mutation took place in the egg of 

 a female from which a male was pro- 

 duced by combination with the sperm 

 carrying no X, no R (or W in our 

 formulae). In other words, if the for- 

 mula for the eggs of the normal female 

 is RX— RX, then the formula for the 

 particular egg that sported will be 

 WX; /. e., one R dropped out of the 

 egg leaving it WX (or no R and one 

 X), which may be written OX. This 

 egg we assume was fertilized by a 

 male-producing sperm. The formula 

 for the two classes of spermatozoa is 

 RX— O. The latter, O, is the male-pro- 

 ducing sperm, which combining with 

 the egg OX (see above) gives OOX 

 (or WWX), which is the formula for 

 the white-eyed male mutant. 



The transfer of the new character 

 (white eyes) to the female (by cross- 

 ing a white-eyed male, OOX to a 



MORGAN 



heterozygous female (Fi)) can there- 

 fore be expressed as follows: 



OX -O (white male) 



RX- OX (Fi female) 



RXOX— RXO - OOXX— OOX 

 Red Red White White 

 female male female male 

 It now becomes evident why we found 

 it necessary to assume a coupling of 

 R and X in one of the spermatozoa of 

 the red-eyed Fi hybrid (RXO). The 

 fact is that this R and X are combined, 

 and have never existed apart. 



It has been assumed that the white- 

 eyed mutant arose by a male-produc- 

 ing sperm (O) fertilizing an egg (OX) 

 that had mutated. It may be asked 

 what would have been the result if a 

 female-producing sperm (RX) had 

 fertilized this egg (OX)? Evidently a 

 heterozygous female RXOX would 

 arise, which, fertilized later by any 

 normal male (RX— O) would produce 

 in the next generation pure red females 

 RRXX, red heterozygous females 

 RXOX, red males RXO, and white 

 males OOX (25 per cent.). As yet I 

 have found no evidence that white- 

 eyed sports occur in such numbers. 

 Selective fertilization may be involved 

 in the answer to this question. 



