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 
rrec; 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 TAO! TC 
RC RRCC RRCc RrCC RrCe 
rough black rough black rough black rough black 
Re RRCc RRcec RrCe Rrec 
rough black rough white rough black rough white 
rC RrcC RrCe m7CC. | rrCc 
rough black rough black smooth black smooth black 
rc RrCce Rrece rrCc TTcc 
rough black rough white smooth black smooth white 
Fia. 41.—Checkerboard showing F2 segregation in the cross, smooth black & 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 Ff, flies of both sexes are long-winged in the reciprocal 
crosses, and in /’, 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 w (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 
Digitized by Microsoft® 
