INHERITANCE OF MINOR VARIATIONS IN INTENSITY. 85 



INHERITANCE OF MINOR VARIATIONS IN INTENSITY. 

 METHODS AND ACCURACY OF GRADING. 



The method of grading has been described on page 60. Every 

 guinea-pig which showed dilute black or yellow in the fur was compared 

 with standard samples of hair within a week of birth. These samples 

 were black , sepia 3 , sepia 6 , and sepia 9 , in the black series, and red , 

 yellow 3 , and cream 6 in the yellow series. Intermediate grades were 

 given by estimate. Grades were taken later in life in many cases in 

 order to determine the relation of age to intensity of pigmentation. 



In interpreting the results, it is important to know the accuracy with 

 which the grading could be done and the difficulties met. In some 

 cases the back and belly are fairly uniform in intensity, but usually the 

 belly is considerably the lighter. Tufts of hair for grading have always 

 been taken as near the middle of the back as possible. 



In some cases the hair is of fairly uniform intensity from base to tip. 

 In most cases, however, the base is very much lighter than the tip. 

 The color at the tip has been used in grading, although extreme varia- 

 tions in the intensity at the base have also been noted. The color at 

 the tip has most to do with the general appearance of the animal. 



The attempt has been made to get both a yellow and a sepia grade for 

 every animal, so that the correlation between the intensities in these 

 series could be determined. This is easy in the sepia and yellow-spotted 

 animals, but in agoutis (where the yellow band of the agouti pattern 

 displaces the sepia near the tip of the hair) determination of the inten- 

 sity of sepia has not been so satisfactory. Several independent determi- 

 nations have been taken in many of these cases. In most cases the 

 same grade was assigned the second time and rarely did the second 

 grade differ from the first by more than one point. 



VARIATIONS IN INTENSE GUINEA-PIGS AND ALBINOS. 



Before discussing the inheritance of variations among dilutes, it will 

 be well to note briefly the range of variation among guinea-pigs which 

 have the intensity factors (CP) . In the B W race the blacks are a very 

 intense black. The base of the hair is only slightly lighter than the 

 tip. In other races, especially the 4-toe stock, the tip of the hair is a 

 dull slaty black and the base a very dull color, often with less pigment 

 than many typical dilutes. The animals have a dull streaky black 

 appearance very different usually from the uniform dark sepia of the 

 darker dilutes. This dull color is not associated with heterozygous 

 albinism. Male M330 was undoubtedly homozygous (CC), having 

 had 9 intense young by albino females and no others; yet he was one 

 of the dullest blacks in stock. On the other hand, nearly all of the 

 intense blacks of the BW race are heterozygous for albinism. 



