THE NATURE OF THE GENE 373 



sub-units (genomeres) of which the gene was supposed to consist. The 

 genomeres were assumed to be of two kinds, producing red pigment or 

 no pigment; an intermediate gene contained a mixture of the two sorts, 

 and the somatic mutation was supposed to result from a reassortment 

 of the genomeres during the division of the gene in mitosis. Demerec 

 pointed out that the hypothesis could not easily be reconciled with the 

 constant mutation rates found by him in mutable genes which he 

 studied; and there is evidence from X-ray work on mutation against 

 the idea that the gene contains equivalent sub-units. 



The segregation of genomeres would give rise to twin patches of 

 coloured and non-coloured tissue. Similar patches have been found by 

 Stem^ in Drosophilas heterozygous for genes affecting the body sur- 

 face, and he has proposed to explain them by a process of somatic 

 crossing-over. When mitosis occurs in a heterozygote Aa, there are 

 four threads AAaa to be distributed to the two daughter cells, and the 

 normal separation of A from A and a from a depends on the sister 

 threads remaining attached to the same centromere until they get on 

 to the metaphase plate and the centromere divides. If a somatic crossing- 

 over occurs, and the two ^'s become attached to different centromeres, 

 there is a chance that they will both pass into the same daughter cell; 

 the division will then produce two homozygous cells AA and aa, from 

 which twin patches of tissue with contrasted characters may arise. 

 Somatic crossing-over seems to be a genuine phenomenon, at least in 

 Diptera; it is not known whether it depends in any way on the close 

 somatic pairing of homologous chromosomes found in this group. 



In other somatic mosaics, still other mechanisms may be at work. 

 Schultz^ has pointed out that high mutation rates are often associated 

 with translocations of part of the inert regions into the active sections 

 of the chromosome, and has suggested an explanation for the mutation 

 process based on the pecuHar mechanical behaviour to be expected in 

 such circumstances (but cf. p. 400). 



3. Position Effects^ 



It was for a long time thought that the arrangement of the genes in 

 the chromosome was completely indifferent and had no influence on 

 their mode of expression. The first evidence against this was produced 

 by Sturtevant,* who showed that two Bar genes adjoining one another 



1 Stem 1936. ^ Schultz 1936. 



' General references: Dobzhansky 1936a, Offermann 1935, 

 * Sturtevant 1925. 



