UNIT-CHARACTERS (GENE MUTATIONS) 27 



A black mouse bearing this pattern is known as black-and-tan 

 (a 1 ) {51). The chocolate mouse with this pattern is a choco- 

 late-and-buff. The genetic relations of A w to a 1 are uncer- 

 tain, except that they behave as allelomorphs of each other 

 and of the agouti gene A. 



When a gray mouse is crossed with a black-and-tan, the 

 offspring are light-bellied grays. Progeny of identical appear- 

 ance may be produced when white-bellied grays are crossed 

 with blacks. Black-and-tan and white-bellied gray have 

 usually been regarded as two additional allelomorphs of 

 the yellow, agouti, non-agouti series (51, 23). But it would 

 seem to be a more probable explanation that white belly 

 depends upon a gene closely linked with the agouti gene. 

 This explanation eliminates the paradox of the black-and- 

 tan, which, when crossed to gray, produces offspring in 

 which the back behaves as a recessive and the belly as a 

 dominant. 



Brown (b) 



The first definite record of the brown mouse is found in a specimen list 

 of mammals in the British Museum by Gray, 1843. 



The brown condition (see Fig. 26) is one in which black 

 pigment throughout the coat, skin, and eyes is replaced by 

 chocolate (54, 110). This is thought to be due to an early 

 interruption of the reaction which regularly produces black 

 pigment. The mouse which would otherwise be wild gray 

 coated becomes a cinnamon when homozygous for the 

 brown gene, and the unticked or non-agouti form becomes 

 pure chocolate in color. 



Blue dilution (d) 



Blues may be thrown by blacks and then breed true. — Bateson, 1903. 



A gray mouse homozygous for blue dilution (9) has a coat 

 exhibiting a washed-out appearance known as blue-gray 

 (53, 51f,, 72, 110). A non-agouti black when homozygous 

 for blue dilution (see Fig. 24) becomes lead colored like a 

 Maltese cat. A chocolate mouse which is also dilute is of a 



