CONTINUOUS VARIATION 



some of the O stock shows a corresponding excess of only 0*73 over 

 the same tester X. Then the X from S gives rise to 2-66 — 0-73 

 or 1-93 more hairs than does the X from O; and this difference 

 cannot be due to secondary action of the B gene, or indeed to any 

 other major gene, for O and S differed in no gene of effect sufficiently 

 large to be followed by the mendelian method. In the same way 

 chromosome II from S exceeded its homologue from O by 

 3*35 — 0'95 or 2-40, and chromosome III by 1-64+ 3 'oS or 

 4*72. Thus, we have observed a total excess of i -93 -f- 2-40-1- 4*72 

 or 9*05 hairs caused by the three large chromosomes of S as com- 

 pared with those of O. 



This method of assaying the action of the various chromosomes 

 by comparison with the tester depends on comparing hair number 

 in flies homozygous for the chromosome under test with that of 

 flies heterozygous for that same chromosome and for the marked 

 chromosome. As we have seen, this prevents us observing the full 

 effect of any gene whose allelomorph in the wild strain is not fully 

 recessive to that in the tester stock. Indeed, fully dominant allelo- 

 morphs must escape detection altogether by such a comparison. 

 Now the Fi females in both crosses, S X tester and O X tester, 

 differed in hair number from those in their parent wild strains. 

 The genes from S and O were therefore not all, or not fully, 

 dominant over their allelomorphs from the tester. These F^ females 

 also, however, differed from one another. So the genes from S and O 

 were furthermore not all, or not fully, recessive to their allelomorphs 

 from the tester. We, therefore, expect our survey to reveal only 

 part of the difference which would be seen between flies homozygous 

 for chromosomes from the wild strains on the one hand and from 

 the tester stock on the other. 



If we assume that dominance is absent or balanced in the two 

 directions (an assumption agreeing well with our general experience 

 of continuous variation), w^e should expect our tests to trace half 

 the difference between each wild type strain and the tester. The 

 comparison of the chromosomes from the two wild-type strains 

 should then reveal half the effects of the genes by which O and S 

 differ in their three large chromosomes. Now females within strain S 

 have on the average 15*7 more hairs than those within O. We have 

 found a grand difference of 9 hairs in our chromosome tests, and 



76 



