UNIT-CHARACTERS (GENE MUTATIONS) 33 



erratic in their appearance and may depend upon several 

 factors, genetic or environmental. Hagedoorn reported a 

 genetic modifier of blue dilution (72). Heterozygosity for 

 brown, pink-eye dilution, or albinism also has a tendency 

 to lighten the coat. Little and Tyzzer {122) believe that 

 susceptibility to development of a certain sarcoma found in 

 the Japanese waltzing mouse is dependent upon three or four 

 independently inherited Mendelizing factors. The evidence 

 is derived from a cross between Japanese waltzing mice and 

 ordinary fancy mice in which the first generation was suscep- 

 tible but in the second generation there appeared a certain 

 percentage of mice of non-susceptible constitution. Other 

 tumor susceptibilities (see Fig. 31) have been demonstrated 

 to be hereditary {121, J>3, 12k, 165). 



Several times in recent years a black-silver strain of mice 

 has thrown occasional individuals bearing a symmetrical 

 lacing pattern of white hairs. Its genetics is complex. Con- 

 genital cataract and stationary pupils have been found to 

 run in certain mouse families. A dilution effective in com- 

 bination with pink-eyed black was found several years ago, 

 but has not been completely analyzed. Recently a separa- 

 tion of the metopic suture, parted frontals (see Fig. 35), was 

 reported (97) as a dominant unit-character. A twisted con- 

 dition of the nasal bones behaving irregularly in heredity 

 was also found (96). 



Another strain of mice produces individuals lacking kid- 

 neys (6), or having lesions of the eyes, head, or feet (120). 

 These are probably related, or expressions of the same 

 process determined by the same genetic factors. 



Nomenclature. The compound breeds of the house mouse 

 are merely combinations of the simple characters already 

 described. It is customary in scientific circles to designate 

 the breeds by analytical names, but the fancier and layman 

 often employ other terms, usually descriptive. In the follow- 

 ing list are given the more common fancier's terms and their 

 analytical equivalents followed by their genetic formulae 

 in terms of mutated genes. 



