372 AN INTRODUCTION TO MODERN GENETICS 



2. Mutable Genes 



Some genes are known with abnormally high mutation rates, which 

 may even reach i per cent of cells. Typically the mutations occur in 

 somatic tissues as well as in germinal cells, so that mosaics or varie- 

 gations are produced; the phenomenon is particularly well known in 

 plants, where flecked or speckled colourations of leaves and flowers 

 are sometimes due to this cause. One of the most fully studied cases 

 is that described by Demerec^ in Drosophila virilis for the locus minia- 

 ture wing. The mutation relations of the different allelomorphs are 

 very complicated; some are quite stable, while others mutate with 

 considerable frequency, each individual mutation-step having a charac- 

 teristic frequency, which may be different in different tissues, but 

 which remains constant over many generations under the same condi- 

 tions. The allelomorphs are classified in five main groups, known as 

 miniature- 1, miniature-2, etc., and each group may contain one or 

 more members, e.g. mt-3a, mt-3^, etc. Members of different groups 

 hardly ever, perhaps never, mutate to one another; they were originally 

 obtained as separate mutations from wild type. Members of the same 

 group have almost the same phenotypic effect, but differ in stability. 

 Most of their mutations are back to the wild type allelomorph, but they 

 also mutate, somewhat more rarely but still comparatively often, to 

 each other. Thus the miniature-3 series has three allelomorphs, mt-3a, 

 mt-3jS and mt-3y, which can be derived by mutation one from the 

 other; mt-3a is unstable and mutates back to wild both in somatic and 

 germinal tissues, while mt-3j8 is stable in somatic tissue but mutates 

 germinally while mt-3y mutates in germinal but not somatic tissue. 

 Other genes are known which increase the mutation rates of these 

 genes both in somatic and germinal tissues. 



The explanation of these high mutation rates is obscure. In many 

 cases we may really be dealing with ordinary genes whose stability is 

 low. But in some cases the mutability is probably produced by special 

 conditions. Eyster^ studied the variegation produced by somatic mu- 

 tation of colour genes affecting the pericarp in maize seeds. There are a 

 series of multiple allelomorphs producing various shades of colour 

 from red (top dominant) to white (bottom recessive). An intermediate 

 gene, producing, say, an orange colour, is found to mutate somatically 

 in both directions simultaneously, giving patches of red and white 

 tissue. Eyster suggested that this might be due to the sorting out of 

 ^ Demerec 1933, 1935. * Eyster 1925, 1928. 



