"Tricolour" and "Non-tricolour" in Basset Hounds : The Law of Ancestral Heredity.
The question of the relation of the yellow and black pigments is raised in the celebrated case of the colours of Basset Hounds. The importance of that subject is due to the fact that it was from a study of the evidence in regard to Bassets that Mr F. Galton was led to enunciate his "Law of Ancestral Heredity" with confidence as one which "appears to be universally applicable to bi-sexual descent." The publication of that paper played a considerable part in the history of modern genetic research and it is necessary that we should consider the facts in some detail.
The colours of Bassets are two, the first spoken of as tricolour, consisting of black and yellow marks on a white ground ; the second, non-tricolour, which differs from the first in having no black. It is said that dogs which cannot be easily referred to one or other of these two types do not occur, and they must certainly be very rare if they exist at all.
Mr Galton's investigation was based on data supplied to him by the late Sir Everett Millais, a keen fancier of the breed. This evidence consisted in records giving the number of offspring of each type which had occurred in families of various compositions. It was thus possible to compare the number of tricolour and non-tricolour dogs produced in the families with the number of the respective types distributed among their pedigrees. Galton's figures indicated that there was a close correspondence between these two numbers, so that it was possible, given the ancestral composition of the families, to predict with considerable accuracy the numerical proportions in which the respective types would appear. According to Galton's system the family was regarded as the production of all the ancestors. Each ancestor was supposed to contribute in his or her degree to this total heritage, the more immediate progenitors contributing more, and the remoter progenitors less, according to a definite arithmetical rule. This rule was that the average contribution of each ancestor was to be reckoned
for each parent 1/4
for each grandparent 1/16
for each great-grandparent 1/64
and so on, the total heritage being thus reckoned as unity. It will be observed that this scheme differs entirely from those based on Mendelian principles, inasmuch as every ancestor is, according to the Law of Ancestral Heredity, supposed to have some effect on the composition of each family in its posterity, and each recent progenitor is regarded as having a very sensible influence on these numbers.
Though no one with a knowledge of practical breeding could entertain the supposition that Galton's Law had the universality of application claimed for it, there was on the other hand no doubt that the Law had successfully expressed a variety of facts in which no order at all had been previously detected.
We have now to consider the meaning of this evidence in the light of modern knowledge. At the time that Galton's views were promulgated nothing was known of segregation. The supposition that any individual, whatever its own characters, was capable of carrying on and transmitting to its posterity any of the characters exhibited by its immediate progenitors, at all events, was generally received without question by biologists. According to that idea the number of classes of individuals differing in respect of their ancestral composition and transmitting powers is to be regarded as indefinitely large, whereas in all cases of sensible allelomorphism the number of classes of individuals three only, two being homozygous and one heterozygous. The difference between the two schemes is thus absolute and irreconcileable.
When Mendelian phenomena were first recognized it was naturally supposed that some classes of cases would be found to conform to the Mendelian scheme and others to the Law of Ancestral Heredity. With the progress of research however almost all the cases to which precise analytical methods have been applied have proved to be reducible to terms of Mendelian segregation ; and of those which have not already been so elucidated some, we may feel confident, if not all, will be eventually shown to be governed by similar rules. In discussing aberrant phenomena like those alleged in regard to the Bassets the first question to be settled is whether the facts are correctly reported. If either type is recessive we should naturally expect this to be the non-tricolour, which is without black. Unfortunately as the non-tricolours are not fashionable there were comparatively few matings between two parents of that colour. Nevertheless 41 dogs, offspring of such matings, are given, of these 20 being tricolour. Though the records were not made by scientific men or with a scientific purpose directly in view it is almost impossible to imagine that all these cases can depend on mistakes, and pending the production of new and direct evidence we must take the records as correct.
In the Theory and Practice of Rational Breeding (London, 1889), pp. 26 and 27, Sir Everett Millais gives one or two more notes bearing on this question. He says that in England there were then two strains of Bassets, the Couteulx and the Lane. "The Couteulx is as a rule a very perfectly marked tricolour, with the tan and black markings deeply accentuated. The Lane hounds, on the other hand, are very weak in markings if they happen to be tricolour, but as a fact they are far more generally found to be lemon and white." In another place he mentions that "in nearly every litter of pure Couteulx there is generally a lemon and white puppy."
If it were not that the genetic relations of yellow and black pigments are, as we have seen, so complicated and uncertain in other types, we might be inclined to attribute the alleged production of tricolours by non-tricolours to imperfect classification of "weak" tricolours, but in dealing with this group of phenomena no such suggestion can be hazarded with any confidence.
The strange fact that yellow mice are never pure naturally occurs to the mind in connection with the evidence as to Bassets. A comparison between the two cases cannot nevertheless be instituted at all easily ; for in the Bassets yellow is evidently not usually a dominant, which it should be if the impurity of the non-tricolour is to be attributed to a state of things comparable with that existing in mice. At present the Basset phenomena must be regarded as definitely unconformable. Perhaps the most probable view of their nature is that they are an illustration of irregular dominance, but this cannot be asserted with much confidence.[/b]