INDEPENDENT MENDELIAN INHERITANCE 



87 



one in nine were homozygous for both dominant factors as may be 

 determined from the checkerboard analysis. Consequently for this com- 

 bination of characters it would be necessary to make extensive tests of 

 the individuals in order to determine their genetic constitutions. Mat- 

 ing those which had been determined to be of the genetic constitution 

 RRCC together would insure the production of a race of rough black 

 guinea-pigs which would breed true for these characters. On the other 

 hand, all those which are smooth white are of the genetic constitution 

 rrcc; they are therefore homozygous and will produce a uniform progeny 

 when bred together. 



Dihybridism in Drosophila. We shall not attempt to follow out the 

 chromosome relations for the guinea-pig hybrid because they are exactly 



RC 



Re 



rC 



RC 



Re 



rC 



FIG. 41. Checkerboard showing Fz segregation in the cross, smooth black X rough white 



in guinea-pigs. 



the same as those in maize. In Drosophila, however, the peculiar rela- 

 tions displayed by the sex-chromosomes gives more striking instances 

 of parallelism of chromosome behavior and factor distribution. The 

 inheritance of white-eye color in Drosophila has already been described 

 in detail. Another character, vestigial wings, shows a different type of 

 inheritance. When vestigial-winged flies are crossed with normal long- 

 winged flies the F\ flies of both sexes are long-winged in the reciprocal 

 crosses, and in F 2 segregation is in the ratio of 3 long : 1 vestigial in both 

 sexes. The factor for vestigial wings, therefore, must be located in one of 

 the pairs of autosomes. We shall call this factor v and its normal allelo- 

 morph in the long-winged race V. The formula for a vestigial-winged 

 white-eyed female then becomes vv (wX) (wX) and for a long- winged red- 

 eyed male VV(WX)Y. 



The chromosome relations involved in crossing a vestigial white 

 female and a long red male are shown diagrammatically in Fig. 42. Two 

 pairs of chromosomes are involved, the sex-chromosomes and a pair of 



