Vol. XLIV. March, 1923. No. 3. 



BIOLOGICAL BULLETIN 



THE TEMPERATURE COEFFICIENT OF A HETERO- 

 ZYGOTE WITH AN EXPRESSION FOR THE VALUE 

 OF A GERMINAL DIFFERENCE IN TERMS OF AN 

 ENVIRONMENTAL ONE. 1 



CHARLES ZELENY. 



It has been shown by Seyster and Krafka that the size of the 

 eye and the number of its ommatidia in " bar-eye " Drosophila 

 varies with the temperature at which the larvae are developed. An 

 increase of one degree Centigrade produces on the average a de- 

 crease of about 10 per cent, in ommatidial number. In ultra-bar, 

 an allelomorph of bar and full, the change is about 8 per cent, per 

 degree. On the other hand, full eye has a much lower temperature 

 coefficient. Counts being made at present by Miss Karrer show 

 a change of only 2 J / 2 per cent, per degree. 



Since the effect upon bar and ultra-bar is so much different from 

 that upon full, it becomes a matter of interest to determine the 

 reaction of the heterozygotes. Are they intermediate in this re- 

 spect as well as in ommatidial number? The results may be ex- 

 pected to throw some light upon the manner of reaction of the 

 genes and on the nature of dominance. 



The present report deals with the ultra-bar heterozygotes. 



The effect of temperature upon ultra-bar homozygotes has been 

 determined by Krafka (1920, p. 416). His values are copied in 

 the next to the lowest line of Table I. From these values the 

 average effect of one degree of change in temperature may be 

 determined as follows: The average ommatidial number is 51.5 at 

 15 ° and 15.8 at 30 . The difference between the logarithms of 

 51.5 and 15.8 divided by fifteen and reduced to its arithmetical 



1 Contribution from the Zoological Laboratory of the University of Illinois. 

 No. 210. 



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