152 



SCIENCE 



[N. S. Vol. XXV. No. 630 



reversion is due to the introduction of a third 

 factor, additional to simple red and simple 

 black. It is evident further that this new 

 third factor, which we will call A (agouti), 

 has been introduced through the red parent, 

 and that as regards this factor, A, some in- 

 dividuals are homozygous (A A) in character, 

 others are heterozygous (transmit it in half 

 their gametes only), while others lack it alto- 

 gether. Further observations show that it is 

 independent in its inheritance of both black 

 and red; it is, in fact, an independent Men- 

 delian character, which can iecome visible 

 only in the presence of both black and red, 

 because it is a mosaic of those two pigments. 



The reversionary individuals (agoutis) do 

 not breed true. They produce offspring of 

 three sorts, agouti, black and red. When 

 mated inter se, they produce young in the 

 proportions, 9 agouti: 3 black: 4 red; when 

 mated with reds (recessives), they produce 

 young in the proportions 1 agouti : 1 black : 

 2 red. 



It has been found by experiment that the 

 jFj agoutis (produced by mating the original 

 agoutis inter se) are of three different sorts. 

 It is probable that they are in reality of four 

 different sorts. The three sorts thus far 

 recognized are (1) agoutis like their parents, 

 which produced the three sorts of young, 

 agouti, black, and red in the proportions 

 already stated; (2) agoutis which in ma tings 

 with recessives (reds) produced only agouti 

 young and red young in equal proportions ; 

 (3) agoutis which in matings with red animals 

 produced no red offspring, but only agouti 

 ones and black ones in equal numbers. The 

 fourth (exi)€cted) class should when mated 

 with reds (or any other color, for that matter) 

 produce only agouti colored young; i. e., 

 should not only resemble wild guinea-pigs in 

 the character of its coat, but should transmit 

 this character to all its young, as wild ones 

 do. This is on the hypothesis already stated 

 that the factor 4 is a distinct Mendelian char- 

 acter. The gametic constitution of the four 

 classes of agoutis would on this hypothesis 

 be: 



1. BA -B, forming gametes B, BA, B, and 

 BA with equal frequency. 



2. BA -BA, forming gametes BA and BA 

 with equal frequency. 



3. BA -B, forming gametes BA and B with 

 equal frequency; all these classes have been 

 proved to exist. 



4. The fourth (expected) class, the fully 

 fixed agouti, known to exist but not yet pro- 

 duced in this experiment, should have the 

 formula BA -BA like class 2 in that A is 

 transmitted in all its gametes, but differing 

 from it in that black also is transmitted in 

 all the gametes, instead of in half of them 

 only. 



But, some one may inquire, how is it that 

 an individual which forms no red gametes (as 

 B'BA or BA'BA) can nevertheless produce 

 agouti young, which character is by hypoth- 

 esis a mosaic of red with black. This is a 

 matter which gave me considerable trouble 

 and made the at first wholly unexpected class 

 3 (B -BA) for a long time perplexing. The 

 explanation is this : Ordinary black individ- 

 uals, while homozygous, are not pure in the 

 sense that they form no pigment but black. 

 They probably always form a certain amount 

 of red and of brown pigment, which is over- 

 looked only because it is interspersed with the 

 more opaque black pigment. If the red pig- 

 ment is segregated, as is the case when the 

 factor A is present, then it becomes visible 

 as the agouti marking of the hair. If this 

 factor. A, is present in both halves of the 

 zygote together with black (and enough red 

 to form the agouti hair tip) a homozygous 

 agouti animal is the result (class 4) ; if J5 is 

 present in both halves of the zygote, but A in 

 one half only, agouti and black young will be 

 produced. 



The existence of a third factor, A, in cases 

 of reversion in coat-character among rodents 

 has been heretofore overlooked merely because 

 it does not represent a distinct pigment or set 

 of pigments, but consists in a particular kind 

 of pigment distribution on the individual 

 hairs. The agouti hair is due to a definite 

 cycle of activity of the hair follicle in form- 

 ing its pigments, first black, then red, then 

 black; the wholly black hair is due to a con- 

 tinuous process of pigment formation with- 

 out alternation in the character of the pig- 



