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SCIENCE. 



[N. S. Vol. XXIV. No. 609. 



zygous, since it may be produced by a cross 

 between yellow and chocolate (see mating 

 c?CX?4), but clear yellow may likewise be 

 produced in tbe same way and may be like- 

 wise heterozygous, as is shown further by the 

 result of mating clear yellows inter se. Clear 

 yellows so mated may produce, as we have 

 seen, both chocolate animals and those of the 

 sooty yellow type. Whether sooty yellows may 

 in turn produce clear yellows, my experiments 

 do not show, but this seems highly probable. 

 Whether these two types differ in gametic for- 

 mula is at present uncertain. The few yellow 

 animals from which I secured young were 

 evidently all heterozygous, like the 81 yellow 

 mice of generation F^ tested by Cuenot. They 

 were also clear yellow, all except one. That 

 one, a sooty yellow female, bore a litter of 

 three young by a black mate ; two of the young 

 were yellow and one black. 



In the hair of clear yellow animals, I have 

 found only yellow pigment granules; but in 

 the hair of a sooty yellow which I examined, 

 chocolate granules occur sparingly, with the 

 yellow ones. 



Yellow heterozygotes may probably also de- 

 velop hlack pigment in their adult coat, when 

 black is the recessive character present. For 

 Dr. G. M. Allen, when studying color-inherit- 

 ance under the writer's direction, obtained a 

 pair of yellow mice, about four weeks old, from 

 a breeder in Washington, D. C, and within a 

 short time these animals began to develop 

 small spots of MacTc upon the back. Unfor- 

 tunately Dr. Allen was soon after forced by 

 the assumption of other duties to discontinue 

 his experiments with the yellow type, and 

 since that time I have been unable to secure 

 material for further study of the matter. 



Cuenot finds in his experiments that all 

 yellow mice tested by him are heterozygous, 

 like blue Andalusian fowls (Bateson, Saun- 

 ders and Punnett). He believes, what seems 

 entirely reasonable, that this is due to infer- 

 tility of gametic unions between yellow and 

 yellow. It will be a matter of interest to see 

 whether further investigations confirm this 

 interpretation, or whether an explanation can 

 be found on the ground of gametic contamina- 



tion, as suggested by Morgan, or of multi- 

 plicity of factors giving rise to yellow, as to 

 the pigmentation of stocks and sweet-peas 

 according to recent investigations of Bateson, 

 Saunders and Punnett (1906). 



In guinea-pigs, as I have elsewhere shown 

 (1905), yellow is recessive in relation to black, 

 yet exceptionally a yellow-coated animal may 

 transmit black pigmentation in about half of 

 its gametes. The black pigmentation so 

 transmitted is always small in amount, being 

 in reality the black constituent of a mosaic 

 predominantly yellow in its composition. In 

 the parent classed as yellow (but really 

 mosaic in nature) the amount of black is 

 extremely reduced, or black may be absent 

 altogether from the hair, though present as 

 peripheral skin pigment, as is frequently the 

 case also in yellow animals when they breed 

 entirely true. Now the small amount of black 

 in individuals predominantly yellow may be 

 considered an impurity, a contamination of 

 the self-yellow character, with black. Such 

 contamination can be brought about by cross- 

 breeding and exists in various different de- 

 grees. For example, (1) yellow-pigmented 

 guinea-pigs always possess black-pigmented 

 eyes. (2) They may possess also black pig- 

 ment in the skin of the extremities (soles of 

 feet, ears) but not in the hair. Cross-breeding 

 with hlacks will increase the amount of such 

 hlacTc pigmentation in the extracted yellows. 

 Animals of these two types are recessives in 

 relation to self black, or to a mosaic of black 

 and yellow; they breed true among them- 

 selves. 



Again, in rabbits, yellow is recessive in re- 

 lation to black, but a yellow rabbit may bear 

 black-tipped hairs on the ears and nose, indi- 

 cating the presence of the black character in 

 a greatly weakened condition. The black 

 impurity, however, is not in a condition of 

 recessiveness. Yellow rabbits never, in my 

 experience, produce black ones. But two yel- 

 low rabbits may produce a brown one, that is 

 a yellow rabbit with sooty 'peripheral' black 

 pigmentation mixed with the yellow. These 

 various facts are mentioned to show that yel- 

 low animals may contain traces at least of the 



