mutation, a new hereditary factor will sud- 

 denly appear, and this new gene will in- 

 fluence the destiny of the species in a small 

 or large degree, depending on the nature of 

 the change. Quite a number of drastic muta- 

 tions are known to have played an important 

 role in animal husbandry. Mutant stocks of 

 domestic animals — such as hornless cattle, 

 short-legged Ancon sheep, and white turkeys 

 — have distinctive features that may be more 

 desirable than those of the original stock 

 (Fig. 26-28). 



A 



Fig. 26-28. The silver fox (A), which has a highly 

 valued pelt, arose by mutation from the wild-type red 

 fox (B). (A, courtesy of the U.S. Department of Agri- 

 culture and the American Museum of Natural History, 

 New York. B, courtesy of the American Museum of 

 Natural History, New York.) 



A mutation gives rise to a new gene that 

 usurps the locus of the old. Consequently the 

 mutant gene becomes allelic to the normal 

 gene from which it has arisen. In diploid 

 cells, only one gene of any allelic pair has 

 been known to mutate at a given time, and 

 the mutant gene may bear either a recessive 

 or a dominant relation to its nonmutated 



Heredity - 51 1 



mate. In any event, the mutant gene imme- 

 diately occupies the old position in a particu- 

 lar chromosome; and thereafter the mutant 

 gene enjoys the same linkage relations as its 

 normal progenitor. In Drosophila many 

 mutations have arisen in pedigreed stocks 

 kept under meticulous observation, so that 

 in each case it is possible to specify which 

 gene is the mutant and which is the normal 

 allele. For example, the gene (zu) that deter- 

 mines white eye mutated from (\V), the red- 

 eye gene; and the gene (b) for black body 

 came from (B), the gray-body allele. Never- 

 theless, little distinction exists between a 

 mutant gene and the normal original, except 

 when the former is known to have arisen from 

 the latter. In wild stocks, wherein no pedi- 

 grees are known, it is not possible to tell 

 which member of an allelic series is the 

 "original" and which are the mutant genes. 

 In fact a study of evolution from the view- 

 point of heredity leads to the conclusion that 

 virtually all genetic differences — between in- 

 dividuals of the same species, and between 

 the different species — have resulted from an 

 age-old series of mutations extending back to 

 the beginnings of life. 



The sudden and unexpected appearance 

 of a new heritable characteristic in a known 

 stock does not always indicate a mutation — 

 in the strictest sense. Such changes sometimes 

 result from chromosomal duplications or de- 

 ficiencies, such as arise from abnormalities in 

 the divisions of the germ cells; they may 

 come from the deterioration of one or more 

 genes in the germ cell line; or, they may 

 indicate a shift in the position of genes in a 

 chromosome — since there is a distinct posi- 

 tion effect, which may modify the behavior 

 of the genes. These phenomena are some- 

 times called "mutations"; but a stricter usage, 

 confining the term "mutation" to the trans- 

 formation of one gene into another, will be 

 adhered to in the present account. 



Mutations are recognized by their pheno- 

 typic effects, but recessive mutants do not 

 produce these effects until the next genera- 

 lion, that is, until homozygous offspring have 



