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THE INttERITANCE OF BLACK, YELLOW AND TORTOISESHELL COAT-COLOUR IN CATS. BY RUTH C. BAZ~tBEI~ (M~s BISBEE), M.Sc., F.L.S. AND E. CATHERINE HERDMAN, M.Sc. (The University of Liverpool, 1926.) THE problem of the inheritance of black, yellow and tortoiseshell coat-colour in cats is both complicated and interesting. Comparatively few careful~y controlled experiments have been carried out, but consider- able data have been collected from brceders, and certain facts have emerged quite clearly, h~ormally the cross black ~ • yellow 3 gives tortoiseshell ~~ and black 33: the reciprocal cross, yellow ~ x black 3, gives tortoiseshell 99 and yellow 33. Tortoiseshell malea occur only as very rare exceptions. These observed facts have been explained in many ditIerent ways. Doncaster (1904) suggested that yeUow is completely dominant over black in the male, incompletely dominant in the female, so giving tortoiseshell daughters and ycllow sons from the cross yellow ? x black 3. Little (1912) pointed out, however, that the reciprocal cross black ~ • yellow 3 gives black sons and tortoiseshell daughters, and that therefore dominance of yellow over black in the male is notan adequate explana- tion of the observed facts. He suggested that both yellow and black are sex-linked, a black o being BB, a black 3 B-, a yellow ~ YY, anda yellow 3 Y-. In the same year Doncaster (1912) concluded that yellow only is sex-linked, and that black is present in all gametes, yellow being completely dominant over all black in the male, incompletely dominant in the female. In 1913 Johannsen put forward a theory almost exactly like that offered by Doncaster in 1904, except that he looked upon black as being dominant to yellow in the male, whereas Doncaster supposed yeUow to be dominant to black. In 1916 Ibsen suggested that tortoise- shell is due to a separate factor, T, which can only act in the presence of black, B, causing ir to be restricted to spots with yellow areas between. He considered black, B, to be dominant to yellow, b, and suggested that normally T is closely linked with b. He supposed both pairs of allelo- morphs, Tt and Bb, to be sex-linked. Wright in 1918, discussing the chemistry of coat-colour, adopted Little's idea of sex-linkage of both 88 Coat-Colour in Cats black and yellow, bar evidently considered yellow to be simply absence of black. In 1919 Little modified bis earlier hypothesis to the extent of placing B, black, in aU X-chromosomes and making Y, yellow, and y, the absence of yellow, a separate pair of sex-linked allelomorphs. He suggested that one dose of yellow is coinpletely epistatic to one dose of black giving yellow in both sexes, but that one dose of yellow to two doses of black gives tortoiseshell. Apart from Doncaster's first suggestion (1904) and the similar one puf forward by Johannsen in 1913, any one of the above hypotheses will account satisfactorŸ for the observed normal facts of inheritance of black, yellow and tortoiseshell. There are, however, a number of exceptional facts of inheritance. A tortoiseshell male does occasionally appear, and somewhat more frequently black daughters ate produced from yellow sires. There ate also one of two eases on record of a yellow female having bred as a tortoiseshell. Many different explanations, based on one or other of the above hypotheses, have been puf forward to account for these excep- tional individuals, but no theory so lar offered has been able to bear the full weight of the recorded facts, and the whole problem of the inheritance of black, yellow and tortoiseshell remains unsettled. (A detailed dis- cussion of the rival theories is given by Bamber in Bibliographia Genetica, Vol. m. pp. 14-44.) Either yellow or black or both ate certainly sex-linked, but the ordinary facts of inheritance can be explained equally well by assuming: (1) that black and yellow are both sex-linked and ate allelomorphs, i.e. two positive factors whieh are alternative to one another; of (2) that both ate sex-linked but ate not allelomorphs, black being present in aH X-chromosomes, one dose of yellow being completely epistatic to one of black; of (3) that both ate sex-linked but are not allelomorphs, yellow being present in all X-chromosomes, one dose of black being completely epistatic to one of yellow; of (4) that yellow only is sex-linked and that black is present in all gametes (i.e. in ah autosome), one dose of yellow being completely epistatic to all black in the male, incompletely so in the female; of (5) that black only is sex-linked and that yellow is present in all RU~H C. BAMBER AND E. CATHERINE I-]ERDMAN 89 gametes, one dose of black being completely epistatic to all yellow in the male, incompletely so in the female. Two of the above possibilities (3 and 5) ate, however, usually dis- counted because at least three matings between yellow cats have been definitely recorded as giving black or tortoiseshell in the offspring (Don- caster, 1913; Whiting, 1918), whereas there is only one doubtful record of two blacks ever having given tortoiseshell in the offspring (Bonhote, 1915). Notwithstanding the contticting evidence ir seems certain that, whatever may be the exact relationship between black and yellow, either there is a difference in dominance in the two sexes or both colours ate sex-linked. The observed results of carefully controlled experiments are few. Little (1912) and Whiting (1915, 1918, 1919) have both recorded results from their own experiments but these involve comparatively few matings. Doncaster (1913) has given the fullest account of the inherit- ance of black, yellow and tortoiseshell but bis data were collected from breeders for the Cat Fancy and were not the results of carefully controlled scientifie experiments. Bonhote (1915) also gives many facts, but again the majority were obtained from breeders, and most of these facts had already been given to Doncaster and were included in his 1913 paper; they ate, therefore, no additional evidence. Thus there is nota very firm foundation of fact on which to erect such a weighty superstructu~e of hypothesis. In the hope of throwing some additional light on the subject we have been carrying out expe¡ with cats during the past four years. The experiments have been chiefly genetical but a little physiological work has also been included. One of us suggested, Bamber 1922, that, ir there is a difference of dominance between black and yellow in the two sexes, ir might be possible to alter the eolour of a male by turning his physiology towards the Ÿ condition. In an attempt to do this, in 1923, three newly born yeUow male kittens were castrated, and, ffom the time they were about four months old, two of these were red with ovarian extraer for a period of six months. The results were entirely negative. We intended to perŸ the same experiments with black males, but, before this was attempted, our genetical work practically proved that there is no difference in dominance in the two sexes, and our physiological work was therefore discontinued. In reporting ou~ breeding experiments, it is important to state clearly that our animals have been kept under conditions which preclude 90 Coat-Colour in Cats the possibility of any inaccuracy in our records. The females have Iived and bred in separate cat-houses with enclosed wired-in grass runs, and have notat any time been allowed liberty. The males have been free to roam the neighbourhood 1, but have each been confined in a separate pen with a single female at the times of mating. Our results, as recorded in this paper, ate, therefore, entirely reliable. Many different crosses have been made, and in each case there have been several litters. The total results of these experiments ate as fol]ows : (a) Black c7 • Black 9--5 Black 8c~; 8 Black 99. (b) Black ~ • Yellow 9--3 Yellow ~~; 3 Tortoiseshell 99. (c) Black c? • Tortoiseshell 9--11 Black ~c~; 8 Yellow c~c7; 9 Black 99; 8 Tortoiseshell 99. (d) Yellow c7 • Yellow 9--see (g) below. (e) Yellow c7 x Black 9--7 Black 8c~; 2 Tortoiseshell ~~2. (f) Yellow c7 • Tortoiseshell 9--1 Black ~; 5 Yellow ~~; 4 Yellow ~; 1 Tortoiseshell 9 ; 1 Anomalous u 9 (4 d) 2 (g) Yellow ~ • Anomalous Yellow 9 (4 d)3--3 Yellow r 2 Yellow 99; 1 Anomalous Yellow ~ (Tortoiseshell?) ~ These results ate given in detail in Table I. Ir seems advisable to do this both because the amount of data from controlled experiments is smaU and because, so often, in condensing results, facts disappear which would be of value to other workers approaching the problem from a different view-point. Also, in such a controversial subject, any fact may prove to have an unsuspeeted signi¡ Reference numbers ate given in the table Ÿ all an~mals which have been used for breeding, so that the interrelationship of our stock can be easily followed. Arable numerals denote individual cats; Roman numerals denote matings. 1 There is, however, no question as to the identity of the tom-cats. They were treated ss personal friends and, as such, were readfly recognisable from other cats which visited the cattery from time to time. z The ~ was yeUow with s minute amount of black on the bsck of her right l¡ loor, snd n¡ therefore be caUed tortoiseshell. She is here elassified as yellow because her breeding behaviour suggests thst her yellow and black do not segregate as they do in a normal tortoisesheU; sbe sppe to be homozygous for yellow in addition to having the smaU amount of black.

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