14 GENETIC STUDIES ON A CAVY SPECIES CROSS. 



B, a factor for black. Black is usually considered as the most com- 

 plete oxidation product of the yellow-brown-black series. 

 Animals lacking this factor to produce black are brown, or 

 can transmit only brown. The latter condition is indicated 

 byb. 



E, a factor for the extended condition of black and brown pigmenta- 

 tion, in distinction from the restricted condition. This factor 

 produces self-colored black or brown animals, whereas its 

 absence, designated by e, is characteristic of the black-eyed 

 or brown-eyed red or yellow coat. 



Rf , a factor for the rough or resetted coat character. Smooth-coated 

 animals lack this factor, and the condition is represented by rf. 



Each color table deals with a single allelomorphic pair, thus keeping 

 the ratios as simple as possible. A number of years ago it was necessary 

 to explain the various kinds of chance ratios, but such discussion may 

 now be advantageously omitted. Likewise it has been shown that the 

 ratios obtained by dealing with two, three, or more pairs of allelomorphs 

 without coupling are the squares, cubes, or higher powers of the simple 

 3 : 1 ratio. Hence, it is obvious that the more complex ratios may be 

 obtained from the simple and we need not deal with all the color char- 

 acters of each animal at one time, but just deal with a single character 

 and its allelomorph in each case. The tables deal with zygotic consti- 

 tution rather than somatic appearance; for instance, an albino may 

 transmit agouti, and therefore be entered in a table in which all the 

 animals entered transmit this factor, irrespective of the somatic colors, 

 or lack of color. 



4. THE AGOUTI CHARACTER IN THE WILD RACE AND IN HYBRIDS. 

 HOMOZYGOUS AGOUTIS IN CROSSES. 



Agouti, the factor which restricts black or brown from the sub-apical 

 portion of the hair and gives a barred appearance, is characteristic of 

 Cavia rufescens. The character is common to all wild rodents. A 

 number of investigations on rats, mice, and rabbits (Cu6not 1903, 1904, 

 1911; Castle 1905, 1905a, 1907, 1907c, 1908, 1909; Hurst 1905; Sollas 

 1909; Morgan 1911) give sufficient evidence that it acts as a unit 

 character, dominant to the non-agouti condition, and segregating in 

 the Fa generation, according to Mendel's law. 



The agouti of Cavia rufescens is of somewhat different appearance 

 from that of Cavia porcellus or Cavia cutleri. It is darker than either, 

 showing a narrower yellow subapical band and more black. There is 

 some variation in this character in the wild rufescens, which accounts 

 for slight differences in systematic descriptions. The belly-hairs of 

 Cavia rufescens vary from yellow to slightly ticked, but in Cavia porcellus 

 the variation is from complete yellow to yellow with a small amount 

 of black at the base. In both species there is a constant relation 



