GENETICS OF SOMATIC CELLS 419 



arose. Also, since development is less simple, the mixtures are much less regular. 

 Mutations occurring in cells which later proliferate give rise to altered spots or sectors 

 whose size and location depends on the time in development when the mutational 

 event has happened and the type of cell lineage in which it arose. One interesting and 

 highly regular type of mixture is the gynandromorph, well known in Drosophila, with 

 part of the animal male and part female. The origin of the anomaly can be attributed 

 to a genetic difference arising between a pair of nuclei produced at an early mitosis 

 in the embryo. Various mixtures can arise but the commonest is the half sider. They 

 can arise by different types of mechanisms which can be proved by both genetic and 

 cytologic evidence and "all the different irregularities occur which will give workable 

 results in each organism." 240 



Another interesting example of somatic mutation in animals is the condition of 

 mosaic fleece in sheep, described by Australian workers. 148 Sheep of a normally 

 short-wooled breed show occasionally areas on which much longer, loosely crimped 

 fleece is growing. Among twenty million animals, 30 have been found that showed this 

 condition. The most interesting feature was the relation between the extent of skin 

 area involved and the frequency of the condition. The percentage area involved could 

 be used to calculate the probable stage of segmentation when the mutation occurred. 

 The results indicate that cells at various early stages of segmentation had about equal 

 chances to undergo the change which occurred in frequencies between 10 ~ 715 and 

 10 ~ 7 - 3 per cell, thus well within the usual range for germ-cell mutations. 



Somatic mutations can be produced by X rays ; this has been first shown in Droso- 

 phila 992 but it holds true for other species as well. 1117 Flecking of pigeon feathers can 

 be induced by X rays and these induced changes are transmitted through successive 

 somatic generations of regenerated feathers after intermittent plucking. A particularly 

 informative investigation has been published recently by Russell and Major 1117 who 

 studied the induction of somatic mutations at specific loci in mice. Since this is the first 

 investigation of its kind on a mammalian organism, it will be described in some detail. 

 Their method was to irradiate embryos heterozygous for four coat-color genes and exa- 

 mine the adults for mosaic patches. As controls, embryos homozygous for the wild-type 

 alleles of the four loci studied were also irradiated, in order to differentiate between 

 developmental effects, leading to abnormal differentiation of pigment cells, and genetic 

 effects. The frequency of such developmental changes was subtracted from the total 

 mosaics in the heterozygous series in order to obtain the frequency of changes due to the 

 expression of the four coat-color recessives. Using this general method, 57 heterozygous 

 first litters of the outcross used (C57BL$ x NB^, heterozygous for the coat-color 

 genes/?: pink-eyed dilution, c ch : chinchilla, d: dilution, and b: brown, the F 2 offspring being 

 phenotypically black) served as unirradiated controls, while the second litter of 65 

 females was irradiated. A total of 60 irradiated litters was examined. Pilot experi- 

 ments indicated that 10^-day-old embryos were particularly suitable for irradiation. 

 This stage was chosen on basis of the consideration that it must be advantageous to 

 irradiate embryos at a stage when the number of pigment-precursor cells was large 



