DILUTION EFFECTS IN CERTAIN EYE COLORS 431 



zygote in an allelomorphic relation. In this case the sex chro- 

 mosome, derived from the mother, contains the eosin allelo- 

 morph (w^), while in the paternally derived chromosome of the 

 homologous pair, the determiner is in the form white (w). 



The formula for the daughters of the above cross becomes then 

 w w^ We suggest calling this individual a 'white-eosin com- 

 pound,' and the use of the term 'compound' as a name for such 

 zygotes as are formed bj^ the union of the mutant factors of a 

 multiple allelomorph system. In the special case of sex chro- 

 mosomal characters in Drosophila, compounding can occur only 

 in the female. 



Eosm is our only eye-color which shows bicolorism and breeds 

 true for that condition. However, another case which we inter- 

 pret as analagous to the lightening of the color of the white-eosin 

 compound is the difference observable between the female of 

 double recessive stock vermilion pink (the 'orange' of previous 

 papers) and the vermilion pink female that is heterozygous for 

 eosin or white. The vermilion pink female, heterozygous for 

 eosin, is lighter in color than the vermilion pink female of stock. 

 This lightening is more marked still in vermilion pink heterozy- 

 gous for white. This case is, as we now interpret it, simply the 

 effect of the heterozygosis in eosin or white, under the favorable 

 condition of the low pigmentation of the vermilion pink eye. 



It soon became apparent that certain factors when heterozygous 

 produced dilution effects in some combinations and not in others. 

 Most of the following experiments have as their object the analy- 

 sis of the conditions of dilution and the definition of the limits to 

 which dilution effects may be observed at either end of the dark- 

 light series. 



Several other problems are dealt with in these experiments, 

 notably the determination of the coupling strength between ver- 

 milion and the white-eosin locus. 



In a few crosses certain flies appeared which are inexplicable 

 under any current Mendelian hypothesis. These have been men- 

 tioned in footnotes to the present paper, and will be dealt with 

 in a separate paper by Mr. Bridges. 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 15, NO. 4 



