ALLELOMORPHIC RELATIONSHIPS IN MENDELISM 149 
When the F'; was grown from such an F’, population Miss Marryat 
obtained excellent agreement with this analysis as is shown by the data 
in Table XXIX. 
Variable Character Expression in the Hybrid.—Sometimes the 
character expression in F, while intermediate displays a range of varia- 
tion extending almost from one parent to the other. This is shown rather 
strikingly in the case of bar eyes in Drosophila (Fig. 69). The bar eye 
factor is a sex-linked mutant factor which is responsible for the pro- 
duction of flies with long narrow eyes instead of the round eyes normal 
for the species. When a female with bar eyes is crossed to a normal 
male the F, all have bar eyes. In the males especially the eyes are 
Fic. 69.—Normal (a, a’) and bar eye (b, b’) of Drosophila; shown in side view and as seen 
from above. (After Morgan.) 
just as narrow as in homozygous races, but among the females some 
may be found which have eyes nearly as narrow as those characteristic 
of homozygous bar eye flies and others which have eyes nearly as round 
as those characteristic of the normal fly. Most of them, however, have 
eyes which display an intermediate effect of the factor. 
This case readily admits of explanation, if the genetic phenomena 
involved are considered. Since the factor for bar eyes is sex-linked we 
may represent the bar-eyed female as (B’X)(B’X), following Morgan 
in employing the primed symbol to indicate a dominant mutant factor. 
The male with normal eyes is then (6’X)Y. When a bar-eyed female is 
mated to a normal male, bar-eyed females and males are obtained in 
F, as shown in the diagram in Fig. 70. 
The F, bar-eyed male obtains his only X-chromosome from the female 
and this chromosome contains the factor for bar eyes. He has exactly 
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