l60 AN INTRODUCTION TO MODERN GENETICS 



which segregation can be detected by the increase in variability. One 

 of the first cases to be interpreted in this way was that of the colour of 

 the grain in wheat. Nilsson-Ehrle^ showed that the red colour is pro- 

 duced under the influence of at least three factors, whose effects are 

 cumulative. In a cross between a race with the three dominants and 

 one with the three recessives the Fi will be intermediate, but the 

 extreme grandparental forms will appear again in the F2, which will 

 therefore be more variable than the Fi. If the two original races each 

 had some dominant extreme factors, these may be combined in some 

 of the F2, with the production of forms more extreme than were the 

 grandparents. Phenomena of this kind are common in investigations 

 on the inheritance of what are frequeiitiy known as "quantitative" 

 characters ; naturally any character may be quantitative if we can find 

 a way of measuring it; but the characters referred to here are those in 

 which more or less continuous variation is observed, so that measure- 

 ment has to be undertaken before the investigation can proceed at all. 

 Standard examples are such things as height or weight in man, length 

 of ear, height, in maize, etc. Similar phenomenon are also very common 

 in crosses between natural species or geographic races (p. 273). 



2. Epistasis 



Another form of interaction between multiple factors is found when 

 one factor is "dominant" over the others, i.e. suppresses their expres- 

 sion. We cannot use the terms dominance and recessiveness in such 

 cases since the factors involved occupy different loci and are not 

 allelomorphs. Bateson^ proposed the words epistasis and hypostasis. 

 Thus in mice the grey coat colour, produced by a factor spoken of by 

 Bateson as G, is epistatic to the black coat colour produced by B, so 

 that the GGBB mouse is grey. 



3. Suppressors'^ 



A gene A epistatic to B completely suppresses the expression of B 

 in the compound AB : it also has an effect of its own. Suppressors are 

 genes which fulfil the first of these conditions but not the second; in the 

 compound with their "suppressee gene" they suppress its expression 

 but have no other effect, and produce no change in the absence of the 

 suppressee. Thus in Drosophila melanogaster there is a factor pr for 



^ Nilsson-Ehrle 1908. 



^ Cf. Bateson 1930. ^ Bridges 1932a, 6, Schultz and Bridges 1932. 



