Pioneers of Mendelian Inheritance in Animals (PMIA)

Pioneers of Mendelian Inheritance in Animals (PMIA) comprises a series of commentaries by Frank Nicholas on papers that illustrate the early discoveries of Mendelian inheritance in animals. The aim is to work through the early 1900s, documenting the excitement and controversies as an increasing number of cases of Mendelian inheritance in animals were reported. Apart from the very first mice papers, PMIA will concentrate on vertebrate species that are included in OMIA, focusing on the major species of domesticated animals.

The PMIA venture was first announced on Mendel Day (8 March) in 2022, and launched on the 8th of July, 2022, two weeks before the bicentenary of Mendel’s birth on the 22nd. The intention is that new commentaries will be added during the rest of 2022. Feedback to on the commentaries, especially where corrections are needed, will be much appreciated!

Thanks to Chris Moran for helpful feedback on the first set of commentaries. Special thanks to Jon Bushell, an archivist at the Royal Society, for providing invaluable information to do with the most likely publication dates of the Reports to the Evolution Committee of the Royal Society

1900 Bateson, W.
Hybridisation and cross-breeding as a method of scientific investigation.
Journal of the Royal Horticultural Society 24: 59-66

View this paper (a scan of the original paper as it appeared in the proceedings of the 1899 London Hybrid Conference, which was volume 24 of the society’s journal, published around 1 April 1900)

This paper by William Bateson is an amazing summary of why hybridisation studies were viewed as a major means of investigating evolution (particularly the "species problem"), in the decades following the publication of Origin of Species in 1859, including the time when Mendel performed his studies with crosses between strains of peas. The key point to remember when reading this paper is that it was written, presented and published before Bateson became aware of Mendel's paper.

As Bateson says at the bottom of page 61: "the whole question of the origin of species turns on the relationship of each species or each variety to its nearest allies" [Bateson's italics are also underlined here, for clarity]. It follows that when "varieties are crossed with their nearest allies, we shall have material from which to answer the main questions of which the Species problem consists" (page 62). 

Bateson, who was then a 37-year-old researcher at Cambridge, delivered this paper on 11 July 1899 to the Royal Horticultural Society’s “Hybrid Conference” in London. Comparing the title of Bateson’s paper with the full title of the conference, namely “International Conference on Hybridisation (the Cross-Breeding of Species) and on the Cross-Breeding of Varieties”, indicates the extent to which Bateson’s address was central to the conference. Indeed, the program for the conference shows that Bateson’s paper was the first to be presented, immediately after the Chairman’s introductory remarks. Interestingly, the speaker who followed Bateson was none other than 41-year-old Hugo de Vries, then a professor of plant physiology at the University of Amsterdam, and one of the three re-discoverers of Mendelism! In his paper entitled “Hybridising of monstrosities”, De Vries actually discusses the mysteries of heredity in the context of Darwin’s theory of pangenesis; a subject on which he had just published an entire book.

To return to Bateson’s paper. More than 30 years after Mendel’s paper was published and less than a year before Mendel’s work became known in the English-speaking world, Bateson provides details of the type of hybridisation and cross-breeding experiments that are needed to shed light on evolution. In the most famous passage of this paper, Bateson states:

"We need particular knowledge of the evolution of particular forms. What we first require is to know what happens when a variety is crossed with its nearest allies. If the result is to have a scientific value, it is almost absolutely necessary that the offspring of such crossing should then be examined statistically. It must be recorded how many of the offspring resembled each parent and how many shewed characters intermediate between those of the parents. If the parents differ in several characters, the offspring must be examined statistically, and marshalled, as it is called, in respect of each of those characters separately. Even very rough statistics may be of value. If it can only be noticed that the offspring came, say, half like one parent and half like the other, or that the whole shewed a mixture of parental characters, a few brief notes of this kind may be a most useful guide to the student of evolution." ​​​​[Bateson's italics in the original are here underlined]

It is very telling that this passage begins and ends with mention of evolution. In the following paragraphs, the word "inheritance" is included, but very much in the context of evolution (pages 63 & 64):

"The essential problem of evolution is how any one given step in evolution was accomplished. How did the one form separate from the other? By crossing the two forms together and studying the phenomena of inheritance, as manifested by the cross-bred offspring, we may hope to obtain an important light on the origin of the distinctness of the parents, and the causes which operate to maintain that distinctness.

Useful contributions to the physiology of inheritance . . . can only be got by an exhaustive study of the results of cross-breeding between various forms whose common origin is not very distant. Such experiments must, besides, be repeated sufficiently often to give a fairly extensive series of observations on which to base conclusions. Anyone, therefore, who wishes to work on these lines would do well to restrict himself to an examination of the transmitting properties of a small group of closely allied varieties or species, and to explore these properties thoroughly within that group. 

Cross-breeding, then, is a method of investigating particular cases of evolution one by one". [Bateson's italics in the original are here underlined]

As he was soon to realise, Bateson was describing exactly what Mendel had actually done decades earlier, unbeknown to just about everyone in the English-speaking world.

Not surprisingly, Bateson had already put his own advice into practice: in the last few pages of this paper, he describes hybridisation experiments he and his colleague Edith Saunders had already done, and were doing, with plants. He also mentions plant hybridisations done by de Vries, who had also provided seeds to Bateson. Emphasising the importance of this work, Bateson states (page 65): "There can be no doubt that, tested by the method of breeding and by study of the transmitting powers, the relation of varieties and species would be shewn in an entirely new light."  And on page 66: "I look to the study of cross-breeding to unravel that extraordinary mass of confusion. I look to this method of investigation to deliver us from the eternal debates on the subject of what is specific rank [i.e. what can be classified as a species] and what is not."

On the final page (66), he continues:

"there is such a thing as species, and we have to find out what are the properties of species. 

It is true that, as to most species and varieties, artificial breeding is impossible, but in numerous cases a beginning can be made. Take merely the phenomenon of local varieties, or local species, or local races, about which such weary discussions have arisen. Each of these offers a particular example of the Evolution problem. In numbers of such cases an investigation of the behaviour on crossing could be practiced, and a very few such experiments would, I venture to predict, do more to establish true views of the relation of species and varieties than the labours of systematists will do in ages."

Still in his evolutionary context, Bateson then proceeds to describe the hybridisations he and Saunders have done, and are doing, with chickens:

"To come much nearer home, we do not know for certain the true relationships—in this special sense—between the varieties of the commonest domestic animals and plants. For example, I have been trying to investigate these relationships between the several kinds of comb in domestic poultry. I have thus far found no one who can tell me for certain what happens when they are crossed. The various forms of comb in our breeds of poultry—simple comb, pea-comb, rosecomb, etc.—are important structural features, which differ from each other very much as many natural species do. The answer generally given is that the result of such crossing is uncertain—that sometimes one result occurs, and sometimes another. This, of course, merely means that the problem must be studied on a scale sufficiently large to give a statistical result. There is here an almost untouched ground on which the properties of specific characters can be investigated."

The results of these chicken hybridisations were published from 1902 onwards, and are discussed in separate commentaries below.

Finally, it is worth noting that, as expected, Mendel is not mentioned by de Vries in his paper at the London Hybrid Conference nor in his 1889 book. However, Mendel is actually mentioned by another author in the conference proceedings! On page 187 of the proceedings, in a paper entitled “Hybridisation viewed from the standpoint of systematic botany” by R. Allen Rolfe (an orchid specialist from the Royal Botanic Gardens at Kew), Mendel is mentioned in passing as a known hybridist of Hieracium (hawkweed), without any reference being cited: “F. Schultes and G. Mendel raised several artificially, and at least seven of them I have seen in the dried state.” As is well known, Hieracium are the plant group to which Mendel turned after completing his work on peas. His Hieracium results were published in 1870.

Overall, this paper by Bateson provides very strong evidence, reinforced by the title of Mendel’s 1866 paper (“Experiments in plant hybridization”) and by the mention of Mendel as a hybridist in the 1899 Hybrid Conference, that Mendel was trying to understand hybrids in the context of evolution; he was not searching for the laws of heredity! It must be stressed that this conclusion is completely consistent with the reality that Mendel’s discovery laid the foundation for genetics. It is, however, arguing that Bateson’s paper and the title of Mendel’s paper and the mention of Mendel in the Hybrid Conference proceedings are just three pieces of evidence supporting the idea that Mendel did not set out to make the discovery for which he is now celebrated. For a far more detailed presentation of the evidence supporting this conclusion, see chapter 2 of the book by Shan (2020).


De Vries, H. (1899) Intrecellulare Pangenesis. Gustav Fischer, Jena. View this book

de Vries, H. (1900) Hybridising of monstrosities. Journal of the Royal Horticultural Society 24: 69-75. View this paper

Mendel, G. (1870) Ueber einige aus künstlicher Befruchtung gewonnenen Hieracium-Bastarde [On Hieracium hybrids obtained by artificial fertilisation]. Verhandlungen des naturforschenden Vereines in Brünn 49: 48-53. View original version View English version

Rolfe, R.A. (1900) Hybridisation viewed from the standpoint of systematic botany. Journal of the Royal Horticultural Society 24: 69-75. View this paper

Shan, Y. (2020) Doing Integrated History and Philosophy of Science: A Case Study of the Origin of Genetics. Boston Studies in the Philosophy and History of Science, volume 320, Springer, Cham, Switzerland. View this book

1900 Bateson, W.
Problems of heredity as a subject for horticultural investigation.
Journal of the Royal Horticultural Society 25: 54-61

View this paper (a scan of the original)

This is the published version of a talk presented by Bateson to the Royal Horticultural Society on 8 May 1900, nine months after the 1899 London Hybrid Conference described in the previous commentary. The printed version was published sometime after 31 October in 1900.

This paper is celebrated for introducing Mendel to the English-speaking world.

Early in the published version, Bateson asks

“How far have we got towards an exact knowledge of heredity, and how can we get further?”

In answer to the first question, he mentions Galton’s (1897a, 1897b) analysis of inheritance of coat colours in Basset Hound dogs, classified by breeders as “tri-colour” and “non-tricolour”, in which Galton had shown that the data were consistent with his (Galton’s) law of ancestral contributions (also known as the law of ancestral heredity), namely that the total heritage of an offspring comprises, on average, ½ from the combined contribution of the two parents, ¼ from the combined contribution of the four grandparents, and so on. Bateson, however, notes that there are many examples of heredity that are not consistent with Galton’s law. The first example Bateson mentions is that “The offspring of a Polled Angus cow and the Shorthorn bull is almost invariably polled”.

Bateson then immediately notes that “quite recently additions to our knowledge of these questions have been made”, referring specifically to two papers by Professor de Vries, published “in the present year”, namely de Vries (1900a, 1900b). Bateson then moves straight on to mention that de Vries refers to a:

“remarkable memoir by Gregor Mendel, giving the results of his [Mendel’s] experiments in crossing varieties of Pisum sativum. These experiments of Mendel's were carried out on a large scale, his account of them is excellent and complete, and the principles which he was able to deduce from them will certainly play a conspicuous part in all future discussions of evolutionary problems. It is not a little remarkable that Mendel's work should have escaped notice, and been so long forgotten.”

Bateson then devotes the rest of his paper to describing Mendel’s results and their implications, concluding

“That we are in the presence of a new principle of the highest importance is, I think, manifest”.

The somewhat dramatic background to this paper, as related by Bateson’s wife in a memoir published after his death (Bateson, B. 1928), is that “He [Bateson] had already prepared this paper, but in the train on his way to town [from Cambridge to London] to deliver it, he read Mendel's actual paper on peas for the first time. As a lecturer he was always cautious, suggesting rather than affirming his own convictions. So ready was he however for the simple Mendelian law that he at once incorporated it into his lecture”.

As appealing as this account is, there is strong evidence against it. For example, Olby (1987) has shown that an eye-witness summary of Bateson’s lecture (Anon., 1900), published just 4 days after the lecture had been presented, has Bateson talking about Galton’s law (as in the published paper), but then mentioning only the first of de Vries’ 1900 papers (1900a) and making no mention of Mendel at all! Noting that de Vries’ first 1900 paper actually makes no mention of Mendel, and considering other supporting evidence, Olby (1987) concludes that if Bateson read any paper for the first time on the train to London, it was de Vries’ first 1900 paper (the one that does not mention Mendel). Then, soon after returning to Cambridge, Bateson read de Vries’ second 1900 paper (the one that does mention Mendel), which led him to then read Mendel’s paper (which was then available in the Cambridge University library, but has since been lost, presumably stolen!) in time to include it as a centrepiece in the published version of his lecture, which appeared no earlier than 31 October 1900 (the date of an addendum inserted by Bateson). This scenario is supported by the fact that Bateson’s published paper also cites the 1900 “re-discovery” papers of Correns and von Tschermak, both of which were not published until after Bateson’s lecture, but in time to be included in the published paper.

Bateson ends this paper by saying:

“there is no doubt we are beginning to get new lights of a most valuable kind on the nature of heredity and the laws which it obeys. It is to be hoped that these indications will be at once followed up by independent workers. Enough has been said to show how necessary it is that the subjects of experiment should be chosen in such a way as to bring the laws of heredity to a real test. For this purpose the first essential is that the differentiating characters should be few, and that all avoidable complications should be got rid of. Each experiment should be reduced to its simplest possible limits.”

As we shall see in following commentaries, Bateson and many others did just that.


Anon. (1900) Lecture [summary of the lecture given by W. Bateson to the Royal Horticultural Society on 8 May 1900]. The Gardeners Chronicle Issue 698: 398. View this report

Bateson, B. (1928) William Bateson, Naturalist: His Essays and Addresses, Together With a Short Account of his Life. Cambridge University Press, Cambridge. View this book

Correns, C. (1900) G. Mendels Regel Über das Verhalten der Nachkommenschaft der Rassenbastarde. Berichte der Deutschen Botanischen Gesellschaft 18: 158-168. View an English translation

de Vries, H. (1900a) Sur la loi de disjonction des hybrides. Comptes Rendus de l'Academie des Sciences (Paris) 130: 845-847. View an English translation

de Vries, H. (1900b) Das Spaltungsgesetz der Bastarde (Vorlaufige Mittheilung). Berichte der Deutschen Botanischen Gesellschaft 18: 83-90. View this paper

Galton, F. (1897a) The average contribution of each several ancestor to the total heritage of the offspring. Proceedings of the Royal Society 61: 401-413. View this paper

Galton, F. (1897b) A new law of heredity. Nature 56: 235-237. View this paper

Olby, R. (1987) William Bateson's Introduction of Mendelism to England: A Reassessment. British Journal for the History of Science 20: 399-420. View this paper

von Tschermak, E. (1900) Über Künstliche Kreuzung bei Pisum sativum. Berichte der Deutsche Botanischen Gesellschaft 18: 232-239. View an English translation

1902 Cuénot, L.
La loi de Mendel et l'hérédité de la pigmentation chez les souris [Mendel's law and the heredity of pigmentation in mice].
Archives de zoologie expérimentale et générale, 3e série 10: 27-30

View this paper as originally published

View a Google-translated English version of this paper

This short paper is dated 12 March 1902. A few weeks later (7 April 1902), a slightly different version (with the same title) was presented at a Séance of the French Academy of Sciences, and was published in Comptes rendus hebdomadaires des séances de l'Académie des sciences 134: 779-781.

This is the first publication to report Mendelian inheritance in an animal.

Lucien Cuénot was a French biologist based in the University of Nancy, who made pioneering contributions to our understanding of genetics. For a review of some of his contributions, see Hickman and Cairns (2003).

In this 1902 paper, Cuénot describes how he commenced breeding mice in 1900, presumably soon after Mendel’s paper was rediscovered, with the specific aim of investigating whether Mendelian inheritance occurs in animals:

“So far, research on the applications of Mendel's law has all focused on the plant kingdom, and it is not known whether this mode of heredity is also found in animals. For the past two years, I have been experimenting on a very favorable material, which allows me to answer in the affirmative.”

Cuénot’s chosen species was mice, and his crosses were between common “house” gray mice and albino mice. All F1 animals were gray. When F1 animals were mated, the resultant 270 F2 generation comprised 198 gray and 72 albino (26.7%). Matings of F2 albinos always produced albino: they were true-breeding. When he mated randomly-chosen pairs of F2 grays, and symbolising the gray allele as “g” and the albino as “b”, he reported that:

“about half of the couples gave me only little grays (189), which proves that one or both parents had only g gametes; the other half of the couples gave me both grays and whites at each litter (162 gray and 57 albino), which proves that each of the two parents had g and b gametes. This time again, according to the probabilities, the number of grays is triple that of albinos (74 and 26%)”

Given that 1/3 of the F2 grays are expected to be homozygous (gg) and 2/3 to be heterozygous (gb), the expected proportions of the three types of matings are:

gg x gg: 1/3x1/3=1/9: expect only gray offspring

gg x gb: 2(1/3x2/3)=4/9: expect only gray offspring

gb x gb: 2/3x2/3=4/9: expect 3:1 gray:albino ratio in offspring

The “about half” of the mating pairs that gave only gray offspring correspond to the first two mating types (expected frequency = 5/9), giving 189 gray offspring and no albinos; the “other half” of mating pairs correspond to the third mating type (expected frequency = 4/9), giving the expected 3:1 ratio of gray: albino.

Interestingly, Cuénot did not stop with the above satisfying results. He then describes a series of matings in which he tested the influence of ancestral generations. He mated F1 grays with albinos. From the resultant offspring, he chose grays and mated them with albinos; and so on for a total of five successive generations. In each generation, the proportion of albino ancestry of the gray mice increased in the sequence 1/2, 3/4, 7/8, 15/16, 31/32. In each case, the mating produced roughly equal numbers of gray and albino mice. Cuénot’s commentary on these results (albeit presented here in an inadequate English translation) are remarkable prescient:

“Now, if there is a disjunction of the characters, we have crossed each time gametes of character b (those of the albino), by gametes b and g (those of the gray); and if the genital gland of the latter contains as many gametes of both types, we must always obtain, at each crossing, as many albinos (b + b), than grey (b+g). The experiments are perfectly consistent, this time again, with the theoretical prediction; for five successive generations, the repeated introduction of white blood, to speak the zootechnical language, in no way diminishes the number of gray mice in the litters.”

Cuénot goes on to say that these results make it possible:

“to predict and understand facts that will seem paradoxical to breeders: an albino mouse, whose ancestors, for a number of generations as great as one wants, were gray, is however an albino of absolutely pure breed who will never present gray atavism [reversion]”.

On the other hand, and just as importantly:

“By crossing two gray mice, each containing (n–1)/n of white [albino] blood, n being as large as one wants, one can obtain grays of absolutely pure breed (g + g) which will never present a return to albinism.”

Cuénot does not mention Galton’s ancestral law (with which Bateson was very familiar; see above commentaries on the year 1900), probably because he was not aware of it. However, these results provide a strong refutation of Galton’s law, and strong validation of Mendelian inheritance in animals.

Cuénot went on to publish many other important genetics results obtained from his mice, some of which are described by Hickman and Cairns (2003).


Hickman, M. and Cairns, J. (2003) The centenary of the one-gene one-enzyme hypothesis. Genetics 163: 839-841. View this paper  

1902 Bateson, W., Saunders, E.R.
Experimental studies in the physiology of heredity. Part II. Poultry.
Reports to the Evolution Committee of the Royal Society 1: 87-124

View this paper

As stated on page 1 of Volume 1 of Reports to the Evolution Committee of the Royal Society, its contents were presented to the committee on 17 December 1901. Based on several addenda, all dated March 1902, and a letter advising that copies had been distributed before 20 May 1902 (Jon Bushell, Royal Society), we can conclude that this volume was published sometime between March and the middle of May 1902, most likely in April 1902.

This paper is the second published report of Mendelian inheritance in animals, and the first to report Mendelian inheritance in non-rodent animals. Specifically, Bateson and Saunders provide evidence for Mendelian inheritance of five trait pairs in chickens.

The first sentence in this paper is “Experiments begun in 1898, carried out by W. Bateson”. However, the names of both Bateson and Saunders appear at the head of every second page of volume 1, including this Poultry paper. On the other hand, some of the text is written in the first person (presumably by Bateson). On balance, Saunders has been here included as an author of this paper.

The starting of these experiments two or so years before Bateson or Saunders or anyone else in the English-speaking world knew of Mendel's paper is explained in Bateson's second 1900 paper, described above: the chicken crosses were started solely with the aim of investigating the species problem in evolution. Of course, as soon as Bateson and Saunders became aware of Mendel's paper, they realised that the results of their hybridisation experiments in chickens and in plants provided ideal resources for testing Mendelian inheritance. Amazingly, the original notebooks recording the chicken crosses and their results have been preserved in Cambridge and can be viewed here

The paper starts by providing details of the breeds used initially, namely Indian Game and White Leghorn, with mention of breeds used subsequently, namely Brown Leghorn, White Dorking and Wyandotte. Observable traits of the breeds are summarised:

Indian Game: pea comb, black mostly) plumage, yellow shank, yellow bill

White Leghorn: single comb, white plumage, yellow shank, yellow bill

Brown Leghorn: same as White Leghorn but with brownish plumage

White Dorking: rose comb, white plumage, white shank, white bill, extra or 5th toe

Wyandotte: rose comb, white plumage, yellow shank, yellow bill

The three types of comb will not be familiar to many readers. Fortunately, Bateson illustrated them in a letter to Erich von Tschermak (one of the three rediscoverers of Mendelism), dated 1st January 1903. (Courtesy of Erich Tschermak collection, archive of the Austrian Academy of Sciences, Vienna; with thanks to Johann Vollmann 11th March 2022)

Details are then provided of pedigrees and results of matings between pairs of the above breeds, of “First Crosses bred together”, and of “Dominant First Crosses bred with Pure Recessive”. Many of the observed numbers are near to expected Mendelian ratios, but there are some exceptions (the latter are discussed under the heading Non-Mendelian Group; pp 110-123).

“Summary of Conclusions from Experiments with Poultry”

(pp. 123-124):
  1. Dominant characters: pea comb, rose comb, extra toe; but crossbred may show some blending. Recessive characters: single comb, normal foot
  1. Some crosses of dominant animals with recessive gave both dominant and recessive, suggesting that the dominants may not be pure-breeding
  1. “In White Leghorns, white plumage is dominant over brown plumage”, but not always
  2. “White shanks and bill are almost wholly dominant over yellow shanks and bill”
  3. “There is no correlation yet perceived between the characters of comb, foot, and colour of plumage”

The molecular basis of the first four of the trait pairs documented by Bateson and Saunders is now known. For details, see:

OMIA 000782-9031: Pea comb in Gallus gallus

OMIA 000884-9031: Rose comb in Gallus gallus

OMIA 000810-9031: Polydactyly in Gallus gallus

OMIA 000373-9031: Feather colour, dominant white in Gallus gallus

For the other chicken trait, skin/shank colour, the likely causal allele has been identified, but not the actual mutation. For details, see:
OMIA 001449-9031: Skin/shank colour, yellow in Gallus gallus

1902 Bateson, W., Saunders, E.R.
Experimental studies in the physiology of heredity. Part III. The facts of heredity in the light of Mendel's discovery.
Reports to the Evolution Committee of the Royal Society 1: 125-160

View this paper

As stated on page 1 of Volume 1 of Reports to the Evolution Committee of the Royal Society, its contents were presented to the committee on 17 December 1901. Based on addenda on pages 124, 138, and 155, all dated March 1902, the volume was published sometime during or after March 1902.

No authors are listed at the start of this paper. However, as with the previous papers in this volume, both names appear at the head of every second page. Both Bateson and Saunders are therefore recorded as authors.

This is a remarkable paper. As detailed below, Bateson and Saunders:

  1. Show that Charles Darwin actually created Mendelian ratios with his snapdragon crosses
  2. Introduce the words allelomorph, heterozygote and homozygote
  3. Propose that the reason why the Andalusian breed of chicken can not breed true is that the breed-standard feather colour corresponds to heterozygosity
  4. Explain why a recessive allele can remain in a population despite being selected against
  5. Explain the common observation of a particular phenotype “skipping a generation” in terms of homozygote recessives not appearing in every generation
  6. Speculate that sex may be inherited in a Mendelian manner
  7. List seven cases of Mendelian inheritance in animals that had been discovered within two years of the rediscovery of Mendelism
  8. Recognise the possibility of multiple alleles in a population
  9. Recognise that the phenotype of a heterozygote may be a “blend” of the phenotypes of its two homozygous parents, but that the gametes produced by that heterozygote will still produce just two types of gametes, corresponding to the allele inherited from each homozygous parent
  10. Clearly explain how the combined effect of the segregation of alleles at many loci will give rise to a “continuous curve”, i.e. they anticipate the resolution of the biometric/Mendelian controversary by Fisher (1918)
  11. Introduce the symbols F1, F2, etc for first-filial, second-filial, etc generations of crosses

Darwin’s Mendelian snapdragon ratios

After a brief introduction, the authors state: “If we turn to any former description of breeding experiments we generally perceive at once that the whole account must be re-stated in terms of Mendel's hypothesis, and that the discussions and arguments based on former hypotheses are now meaningless. As an illustration we may take the account which Darwin gjves of his experiments with peloric Antirrhinum.” The authors are here referring to Darwin’s well-known hybridising experiments with snapdragons, described in one of the chapters on inheritance in volume 2 of his book entitled The Variation of Animals and Plants Under Domestication. Bateson and Saunders specifically refer to an 1885 reprint of the second (1875) edition, where, on page 46, Darwin reports reciprocal crosses of peloric (symmetrical flower) and common (asymmetrical flower) snapdragons (Antirrhinum majus). In the two large beds of first-cross seedlings, Darwin reports that “not one was peloric”. Darwin then reports that “The crossed plants, which perfectly resembled the common snapdragon, were allowed to sow themselves, and out of a hundred and twenty-seven seedlings, eighty-eight proved to be common snapdragons, two were in an intermediate condition between the peloric and normal state, and thirty-seven were perfectly peloric, having reverted to the structure of their one grandparent.” Darwin was a bit puzzled by these results. In contrast, the Mendelian interpretation of Bateson and Saunders is very clear: peloric is dominant to common, and selfing of F1 plants produces a ratio of 88:37 (neglecting the 2 intermediates) which to them was close enough to the expected 3:1, and to modern readers is not significantly different from 3:1 (P = 0.23).

Introducing key genetic words

Bateson and Saunders then proceed to introduce and define some foundational genetic words (on page 126):

“[The] purity of the germ-cells [of hybrids], and their inability to transmit both of the antagonistic characters, is the central fact proved in Mendel’s work. We thus reach the conception of unit characters existing in antagonistic pairs. Such characters we propose to call allelomorphs, and the zygote formed by the union of a pair of opposite allelomorphic gametes, we shall call a heterozygote. Similarly, the zygote formed by the union of gametes having similar allelomorphs, may be spoken of as a homozygote.”

“Simple and convincing explanations of many facts hitherto paradoxical”

The authors then make the point that Mendelism provides explanations for observations that until now have been puzzling. One example they give concerns the Andalusian chicken, a breed whose “colour is a blue-grey mixed with dull black” and which does not breed “true to colour”. Correctly, they conclude that “There is, therefore, a strong probability that the Andalusian is a heterozygote”. See OMIA 001154-9031 : Plumage pattern (Blue Andalusian) in Gallus gallus

The authors then discuss, on pages 132-136, what we would now describe as the extent to which recessive alleles remain in a population even though there is selection for the dominant phenotype, i.e. selection against a recessive is relatively inefficient, because recessive alleles remain ‘hidden’ in heterozygotes. Even complete selection against a recessive has its limitations: “a recessive allelomorph may even persist as a gamete without the corresponding homozygote having ever reached maturity in the history of the species [the original authors’ emphasis]”. Examples of the resultant irregular by persistent appearance of “rogues” include the repeated but irregular appearance of horned goats in a polled flock. On page 134, the authors even argue that the only way to successfully select against a recessive is to progeny-test individuals with the dominant phenotype, by selfing (if possible) or (if not) by crossing with a recessive homozygote. This section shows remarkable insight into the population-genetic implications of Mendelism. On pages 136-137, the authors explain how Mendelism also explains the oft-observed phenomenon of a particular phenotype ”skipping a generation”.

Another point raised by Bateson and Saunders in this section (page 138) is the possibility that the inheritance of sex may be also be Mendelian: "There is already a considerable body of evidence in favour of the view that difference of sex is primarily a phenomenon of gametic differentiation."  

Mendelian traits

Starting on page 139, the authors present a list of pairs of traits “in which the phenomenon of allelomorphism [Mendelism] has either been actually proved or may be safely inferred for the published records”. In addition to 19 plant cases, the list includes seven animal cases:



Further information

Normal/waltzing habit


Presence/absence of extra toe


OMIA 000810-9031: Polydactyly in Gallus gallus

Pea comb/single comb


OMIA 000782-9031: Pea comb in Gallus gallus

Rose comb/single comb


OMIA 000884-9031: Rose comb in Gallus gallus

White/yellow shanks


OMIA 001449-9031: Skin/shank colour, yellow in Gallus gallus

White/brown coloration


OMIA 000373-9031: Feather colour, dominant white in Gallus gallus

Polled/horned breeds

cattle and probably goats

OMIA 000483-9913: Polled/Horns in Bos taurus

OMIA 000483-9925: Polled/Horns in Capra hircus


Evidence for the mouse case had been published in two papers by Georg von Guaita (1898 and 1900), in neither of which is Mendel mentioned. Bateson and Saunders acknowledge Correns "For reference to this interesting case". It is unclear whether it was Correns or Bateson and Saunders who reanalysed von Guaita's waltzing data and concluded that it showed Mendelian inheritance.

Evidence for the five chicken cases was provided by Bateson and Saunders earlier in this same report (pages 87-124)

For the case of polled vs horned, the authors provide the following footnote (on pages 140-141):

“It is almost certain that absence and presence of horns are allelomorphic characters. In England there are three principal breeds of polled cattle—the Aberdeen-Angus, Galloway and the Red Polled. The first two are black, the last red. Between these and the horned breeds crosses are annually made in large numbers. This is especially the case with the Angus, from which great numbers of cross-bred cattle are annually bred for the meat market. These are usually Angus-Shorthorn crosses, but other horned breeds are occasionally used. The cross between a pure Angus and a pure Shorthorn is almost always a blue-grey without horns. Generally the horns are represented by loose corns of horny material, sometimes embedded in the skin and not rarely hidden by the hair. Such “scurs”, as they are called in the north, are objected to in the pure polled breeds and are mostly absent.”

The authors then provide data from the Smithfield Club Cattle Shows between 1888 and 1901, noting that “The animals are classified according to the Catalogue”:


Polled offspring

Horned offspring

Polled Angus/Galloway/Red Polled x horned



First-cross x pure polled animal



First-cross x horned



Reasonably, the authors conclude “When allowance is made for the very rough materials out of which these figures come, it is clear that the facts cannot be very far from the Mendelian expectation”.

On page 152, in the first paragraph the authors raise the possibility that more than two allelomorphs (i.e. multiple alleles) can exist in a population “yet each zygote can . . . bear only two”.

“Non-Mendelian cases”

On this same page, the authors then begin a discussion of “Non-Mendelian Cases”, citing some of their results with poultry, and concluding “It is certain that these exceptions at all events indicate the existence of other principles which we cannot yet formulate”.

The authors then proceed to consider “Blending Inheritance”, in which “heterozygotes may show either of the parental characters discontinuously, or various blends between them”, noting that “the gametes which composed the heterozygotes may still be pure in respect of the parental characters”. Indeed, “The degree of blending in the heterozygotes has nothing to do with the purity of the gametes”.

Extending their discussion of blending inheritance by mentioning continuous characters such as human height, it is remarkable to see that Bateson and Saunders fully anticipate R.A. Fisher’s 1918 resolution of the biometric/Mendelian controversy: for

“a typically continuous character, there must certainly be on any hypothesis more than one pair of allelomorphs. There may be many such pairs . . .  If there were even so few as, say, four or five pairs of possible allelomorphs, the various homo- and hetero-zygous combinations might, in seriation, give so near an approach to a continuous curve, that the purity of the elements would be unsuspected, and their detection practically impossible. Especially would this be the case in a character like stature, which is undoubtedly very sensitive to environmental accidents.”

Some readers will recall that in his 1918 paper, Fisher attributes the multifactorial concept to Yule (1902) and Pearson (1904). Intriguingly, Yule's 1902 paper is actually a review of two of Bateson's 1902 publications, namely Volume 1 of Reports to the Evolution Committee of the Royal Society (the volume that includes the above Bateson and Saunders multifactorial quote); and Bateson's (1902) book Mendel's Principles of Heredity: A Defence, which, as mentioned in the commentary on Castle's 1903 Mendel's law of heredity papers (below), also contains a description of the multifactorial concept. In his 1902 review, Yule even acknowledges (on page 232) Bateson's consideration of "continuity of variation". But despite this, Yule never acknowledges Bateson's multifactorial concept when developing his own version, i.e. the version later acknowledged by Fisher. Equally mysterious is why Fisher did not acknowledge Bateson, with whose work he was very familiar.

The final section of this paper considers “Galton’s Law of Ancestral Heredity in relation to the new Facts”. Galton’s law was first fully published in 1897. A comprehensive assessment of this law has been given by Bulmer (1998), whose summary of the law is “Galton's ancestral law states that the two parent contribute between them on average one-half of the total heritage of the offspring, the four grandparents one-quarter, and so on. He interpreted this law both as a representation of the separate contributions of each ancestor to the heritage of the offspring and as a multiple regression formula for predicting the value of a trait from ancestral values.” In the present (OMIA) context, the importance of Galton’s law is that in his 1897 paper, the only data used by Galton to illustrate his law comprised coat-colour records from Basset Hound dogs. It was quite an extensive data set. As summarised by Galton: “a total of 817 hounds of known colour, all descended from parents of known colour. In 567 of these 817, the colours of all four grandparents were also known.  . . .  In 188 of the above cases the colours of all the eight great-grandparents were known as well.” From his analysis, Galton concluded that the inheritance of the two “recognized varieties of colour”, namely “tricolour” and “lemon and white” (the latter designated “non-tricolour” by Galton) were in very good agreement with his law. Without testing Galton’s data for evidence of Mendelian inheritance, Bateson and Saunders make the general point that the results of matings of heterozygotes and of heterozygotes with homozygous recessives are expected to be consistent with both Mendelian inheritance and with Galton’s law. It remains for someone to investigate the extent to which Galton’s Basset Hound data is consistent with Mendelism.


In their conclusion (page 159), the authors state:

“We have now sketched the principal deductions already attained by the study of cross-breeding, and we have pointed out some of the results now attainable by that method. The lines on which such experiments can be profitably undertaken are now clear and a wide field of research is open.”

In a footnote they then introduce a terminology that has become universal:

“It is absolutely necessary that in work of this description some uniform notation of generations should be adopted.  . . .  in future we propose to use a system of notation modelled on that used by Galton in ‘Hereditary Genius’. We suggest as a convenient designation for the parental generation the letter P.  . . .  The offspring of the first cross are the first filial generation F. Subsequent filial generations may be denoted F2, F3, &c. Similarly, starting from any subject-individual, P2 is the grandparental, P3 is the great-grandparental generation, and so on.”


Bateson, W. (1902) Mendel's Principles of Heredity: a Defence. Cambridge: Cambridge University Press. View this book

Bulmer, M. (1998) Galton's law of ancestral heredity. Heredity 81: 579-585. View this paper

Darwin, C.R. (1875) The variation of animals and plants under domestication. London: John Murray. 2nd edition. Volume 2. View this book

Fisher, R.A. (1918) The correlation between relatives on the supposition of Mendelian inheritance. Transactions of the Royal Society of Edinburgh 52: 399-433. View this paper

Galton, F. (1897) The average contribution of each several ancestors to the total heritage of the offspring. Proceedings of the Royal Society 61: 401-13. View this paper

Guaita, von G. (1898) Versuche mit Kreuzungen von verschiedenen Rassen der Hausmaus. Berichte der Naturforschenden Gesellschaft zu Freiburg I.B. 10: 317-332. View this paper

Guaita, von G. (1900) Zweite Mittheilung uber Versuche mit Kreuzungen von verschiedenen Hausmausrassen. Berichte der Naturforschenden Gesellschaft zu Freiburg I.B. 11: 131-138. View this paper

Pearson, K. (1904) Mathematical contributions to the theory of evolution. XII. On a generalised Theory of alternative Inheritance, with special reference to Mendel's laws. Philosophical Transactions of the Royal Society of London. Series A 203: 53-86. View this paper

Yule, G.U. (1902) Mendel’s Laws and their probable relations to intra-racial heredity. New Phytologist 1: 193-207; 222-237. View this paper


1903 Castle. W.E.
Note on Mr. Farabee's Observations.
Science 17: 75-76

View this paper

This was published on 9 January 1903. It contains the first report that albinism is a Mendelian recessive trait in guinea pigs and rabbits.

This is a very brief letter to the editor, in response to an adjacent letter from William Farabee providing evidence that albinism is recessive in humans. Castle sets the scene in his second sentence: “The point needs emphasizing that albinism in mammals in general is a recessive character in the sense of Mendel’s law.” [Castle’s italics]. He continues:

“Last winter [i.e., late 1901 or early 1902] in my lectures on heredity, . . . I showed from the statistics published by von Gaeta in 1900 that albinism in mice is a recessive character. This result has been confirmed by Mr. G. M. Allen, who has been carrying on breeding experiments with mice, under my direction, for the past two years.”

The von Guaita 1900 paper mentioned by Castle is one of the two von Guaita papers mentioned by Bateson and Saunders 1902 (Reports to the Evolution Committee of the Royal Society 1: 125-160; see commentary above) as containing data consistent with Mendelian inheritance of the waltzing trait in mice. This reflects the fact that von Guaita recorded both the waltzing trait and albinism in his mice crosses. Not being aware of Mendel, he did not put his data to any Mendelian test.

Castle then goes on to say:

“During the last few months I have been able to demonstrate experimentally that albinism is a recessive character likewise in guinea-pigs and rabbits.”

This being a brief letter to the editor, there is no room for Castle to present his actual data, which was published subsequently, in 1903 and, more fully, in 1905.

Castle is careful to note that the albinism he is reporting is not the same trait as the white plumage reported by Bateson (and Saunders; 1902 Reports to the Evolution Committee of the Royal Society 1: 87-124; see commentary above) to be dominant in chickens.

For a summary of knowledge about albinism in guinea pigs and rabbits, see:

OMIA 000202-10141 : Coat colour, oculocutaneous albinism type I (OCA1), TYR-related in Cavia porcellus

OMIA 000202-9986 : Coat colour, oculocutaneous albinism type I (OCA1), TYR-related in Oryctolagus cuniculus


Castle. W.E., Allen, G.M. (1903) The heredity of albinism. Proceedings of the American Academy of Arts and Sciences 38: 602-622. View this paper

Castle, W.E. (1905) Heredity of coat characters in guinea-pigs and rabbits. Carnegie Institute Publication Issue 23: 1-78. View this publication

von Guaita, G. (1900) Zweite Mittheilung uber Versuche mit Kreuzungen von verschiedenen Hausmausrassen. Berichte der Naturforschenden Gesellschaft zu Freiburg I.B. 11: 131-138. View this paper

1903 Castle, W.E.
Mendel's law of heredity.
Proceedings of the American Academy of Arts and Sciences 38: 535-548

View this paper

This paper was published on 14 January 1903.

It is a review of Mendelian knowledge as it existed at the end of 1902. In addition to strongly endorsing Mendelism, Castle strongly endorses Bateson and Saunders’s explanation of how variation in continuous traits can be due to the segregation of many Mendelian factors, describing the explanation as a “pregnant suggestion”.

Castle starts with an enthusiastic endorsement of Mendelism:

“What will doubtless rank as one of the great discoveries in biology, and in the study of heredity perhaps the greatest, was made by Gregor Mendel, an Austrian monk, in the garden of his cloister, some forty years ago.  . . .  Mendel’s law was rediscovered independently by three different botanists engaged in the study of plant-hybrids —de Vries, Correns, and Tschermak, — in the year 1900.”

Castle then acknowledges the key contribution of Bateson:

“It remained, however, for Bateson, two years later, to point out the full importance and wide applicability of the law. This he has done in two recent publications with an enthusiasm which can hardly fail to prove contagious. There is little danger, I think, of Mendel’s discovery being again forgotten.”

The two Bateson publications cited by Castle are the 1902 volume 1 of the Reports to the Evolution Committee of the Royal Society (see commentaries above) and Bateson’s 1902 book Mendel’s Principles of Heredity: a Defence” (Cambridge University Press, Cambridge). Although this book is very important in the history of Mendelism, there is no separate commentary on it in the PMIA collection because it contains nothing specifically relevant to OMIA species.

Returning to Castle, on page 539 he mentions results of his student Mr G.M. Allen, who obtained the expected Mendelian ratio of grey versus white (meaning albino) in F2 mice; and who has shown that white/albino mice, being recessive, breed true.

In passing, on page 544, there is a brief mention of results obtained by Castle and Allen confirming that the dancing trait in mice (mentioned as waltzing by Bateson and Saunders (1902; see above) is recessive (and may be associated with reduced viability).

Importantly, in relation to continuous traits, on pages 545-546 Castle states that:


“Bateson [referring to Bateson’s 1902 book and Bateson and Saunders (1902; see above)] makes the pregnant suggestion that even cases of continuous variation may possibly prove conformable with Mendelian principles. Take, for example, the height of peas. It has been found in certain crosses of a tall with a dwarf variety of pea, that the hybrid has an intermediate height. Now, if the hybrid produces pure germ-cells, dwarf and tall respectively, in equal numbers, the next generation will consist of three classes of individuals, dwarf, intermediate, and tall, in the proportions, 1 : 2 : 1. But if each of the original characters should undergo disintegration, we might get a dozen classes, instead of three, resulting in a practically continuous frequency-of-error curve.” 

Bateson’s 1902 book includes (on pages 31 and 32) a similar explanation as in the other Bateson reference mentioned by Castle (volume 1 of Reports), of how variation in continuous traits can be due to the segregation of many Mendelian factors:

“In the case of a population presenting continuous variation in regard to say, stature, it is easy to see how purity of the gametes in respect of any intensities of that character might not in ordinary circumstances be capable of detection. There are doubtless more than two pure gametic forms of this character, but there may quite conceivably be six or eight. When it is remembered that each heterozygous combination of any two may have its own appropriate stature, and that such a character is distinctly dependent on external conditions, the mere fact that the observed curves of stature give " chance distributions" is not surprising and may still be compatible with purity of gametes in respect of certain pure types.”

So, we now have direct evidence that on both sides of the Atlantic, the multufactorial conceptual solution to the biometrical/Mendelian controversary was recognised as early as 1902!

As mentioned in the earlier commentary on Bateson and Saunders (1902; Part III), it remains a mystery as to why neither Fisher (1918) nor Yule (1902) acknowledged Bateson; and now we can add Castle (1903) as well.


Bateson, W. (1902) Mendel’s Principles of Heredity: a Defence. Cambridge University Press, Cambridge. View this book

Bateson, W., Saunders, E.R. (1902) Reports to the Evolution Committee of the Royal Society 1: 1-160. View this volume

1903 Castle. W.E.
The heredity of sex.
Bulletin of the Museum of Comparative Zoology 40: 187-218

View this paper

The publication date of this paper is January 1903.

In this paper, Castle builds on the speculation of Bateson and Saunders (1902, Part III; see above) by evaluating available evidence on sex, from which he concludes strongly that sex is inherited in a Mendelian manner.

On page 191, Castle again unambiguously embraces Mendelism:

“Perhaps the greatest discovery, made in the study of heredity is what is commonly known as Mendel’s Law.”

He continues:

“Bateson and Saunders [1902, Reports to the Evolution Committee of the Royal Society 1: 125-160; see commentary above] in a recent paper suggest that sex may be inherited in accordance with that law. In the light of this suggestion certain phenomena of sex are in this paper examined, and found to have their almost perfect parallels in recognised Mendelian phenomena. In consequence we get a new point of view from which to study the phenomena of sex, and many of its long-time mysteries find ready explanation.”

After evaluating the available evidence, Castle concludes (page 214):

“Sex is an attribute of every gamete, whether egg or spermatozoon, and is not subject to control through environment. It is inherited in accordance either with Mendel’s law of heredity [among dioecious animals and plants] or [in the case of hermaphroditic animals and plants] with the principle of mosaic heredity.”


1902 Bateson, W., Saunders, E.R. (1902) Experimental studies in the physiology of heredity. Part III. The facts of heredity in the light of Mendel's discovery. Reports to the Evolution Committee of the Royal Society 1: 125-160. View this paper

1903 Castle. W.E., Allen, G.M.
The heredity of albinism.
Proceedings of the American Academy of Arts and Sciences 38: 602-622

View this paper

This paper was received on 11 March 1903. This paper includes the first mention of Mendelian inheritance in a fish, namely albinism in trout.

The paper begins:

"This paper contains a preliminary statement of certain results of breeding experiments with mice, guinea-pigs, and rabbits, which have been conducted in the Zoological Laboratory of Harvard University during the last two and a half years. The experiments with mice are the work principally of the junior author [Allen]; those with guinea-pigs and rabbits, of the senior author [Castle]."

This tells us that Castle and Allen started their investigations of Mendelian inheritance in mice, guinea pigs and rabbits very soon after the rediscovery of Mendel's results. Importantly, this paper contains some (but not comprehensive) evidence in support of Castle's claims in earlier 1903 papers of Mendelian inheritance of albinism in those three species (see previous commentaries). The comprehensive evidence was published by Castle (1905).

The authors start with a comprehensive review of the Mendelian results for albinism in mice obtained by Cuénot (1902; see previous commentary). 

The evidence provided in this paper is that, for guinea pigs, rabbits, repeated matings of albino x albino (irrespective of parentage), produces only albino offspring. Interestingly, they cite similar evidence in trout from the United States Fish Commission, leading them to conclude that albinism in trout is also inherited in a Mendelian recessive manner. 

The remainder of the paper is devoted mainly to detailed discussion of various controversial coat-colour results in mice. 

The main conclusion is

"Complete albinism, without a recorded exception, behaves as a recessive character in heredity." 


Castle, W.E. (1905) Heredity of coat characters in guinea-pigs and rabbits. Carnegie Institute Publication Issue 23: 1-78. View this publication

1903 Castle. W.E.
Mendel's law of heredity.
Science 18: 396-406

View this paper

This is a revised version (published on 25 September 1903) of the paper of the same title published nine months earlier, in January 1903 (see commentary above).

This paper includes the first published mention of the Mendelian inheritance of rough/smooth hair in guinea pigs.

Sometime during 1903, Castle discovered that Cuénot had reported the Mendelian inheritance of gray/white (albino) in mice the previous year (see commentary above on Cuénot, 1902). The results of Allen therefore independently confirmed Cuénot’s results.

On page 400 of this paper, as in his 9 January 1903 paper (see a previous commentary), Castle implies that he has data showing similar results in other animal species: “Among domesticated guinea-pigs, as among mice and rabbits, albinism is recessive with respect to pigmented coat”. As we have seen in a previous commentary, the data supporting this claim were published by Castle (1905).

Castle then implies he has evidence that the Abyssinian or rough coat of guinea-pigs is a Mendelian trait, dominant to smooth coat. He then proceeds to consider at length the joint Mendelian segregation of the two pairs of traits, namely pigmented/white and rough/smooth. But no data are presented.

The remainder of the paper is very similar to the earlier (January 1903) version.


Castle, W.E. (1905) Heredity of coat characters in guinea-pigs and rabbits. Carnegie Institute Publication Issue 23: 1-78. View this publication

Cuénot, L. (1902) La loi de Mendel et l'hérédité de la pigmentation chez les souris [Mendel's law and the heredity of pigmentation in mice]. Archives de zoologie expérimentale et générale, 3e série 10: 27-30. View an English translation of this paper

1903 Castle, W.E.
The heredity of 'Angora' coat in mammals.
Science 18: 760-761

Pubmed reference: 17844480. DOI: 10.1126/science.18.467.760.b.

View this paper

This paper was published at the end of 1903, on 11 December.

It includes the first report of Angora or long hair coat as a Mendelian recessive trait in guinea pigs and in rabbits.

The paper is too brief to include the supporting data, which were published subsequently by Castle (1905).

For current knowledge of this trait in those two species, see:

OMIA 000439-10141: Hair, long in Cavia porcellus

OMIA 000439-9986: Hair, long in Oryctolagus cuniculus

The rest of this short paper is devoted to so-called Oregon Wonder Horses, which, according to a newspaper report that Castle recalls, “had mane and tail fourteen feet long”, and which Castle suggests may be the equine equivalent of long hair in guinea pigs and rabbits, and, indeed, in most mammals. Castle’s plea for information on these horses is answered by Davenport six weeks later on 22 January 1904 (see commentary below).


Castle, W.E. (1905) Heredity of coat characters in guinea-pigs and rabbits. Carnegie Institute Publication Issue 23: 1-78. View this publication

Davenport, C.B. (1904) Wonder horses and Mendelism. Science 19: 151-153. View this paper

1904 Davenport, C.B.
Wonder horses and Mendelism.
Science 19: 151-3

Pubmed reference: 17839722. DOI: 10.1126/science.19.473.151.

View this paper

Despite its distinctly equine title, this paper is actually the first to claim Mendelian inheritance of polydactyly in cats!

This brief paper starts with a response to Castle's query about long-haired "Wonder" horses, published in the same journal just a few weeks earlier (see previous commentary). Davenport describes a horse whose photograph he has been sent by a Mr Rutherford: 

"The photograph shows a Morgan horse probably about five years old with a double main which trails on the ground on either sde for a distance of two feet. The tail trails on the ground for a distance of about sic to eight feet."

After examining pedigree information also provided by Mr Rutherford, for evidence of Mendelian inheritance of the long hair, Davenport concludes that "The data are, as we see, insufficient to decide the matter."

Davenport then mentions that

"The question of Mendelian behavior of animal mutations has long interested me and I have collected some statistics bearing on the subject."

Among traits that he then goes on to describe, the PMIA-relevant trait is polydactyly in cats. By analysing limited family data on polydactylous cats published in Nature by Poulton (1883), Davenport concludes "This case is easily explained on Mendelian principles" by assuming polydactyly to be dominant. In reality, the data are so limited that the evidence is not strong. However, Davenport's conclusion for this trait turns out to have been correct.

For current knowledge on polydactyly in cats, see:

OMIA 000810-9685 : Polydactyly in Felis catus


Poulton, E.B. (1883) Observations on Heredity in Cats with an Abnormal Number of Toes. Nature 29: 20-21. View this paper

1904 Doncaster, L.
On the inheritance of tortoiseshell and related colours in cats.
Proceedings of the Cambridge Philosophical Society 13: 35-38

A commentary on this paper will appear in due course

1904 Hurst, C.C.
Mendel's discoveries in heredity.
Transactions of the Leicester Literary and Philosophical Society 8: 121-134

A commentary on this paper will appear in due course

1905 Davenport, C.B.
The origin of black sheep in the flock.
Science 22: 674-5

Pubmed reference: 17797580. DOI: 10.1126/science.22.569.674.

View this paper

This paper is the first to report Mendelian inheritance of black sheep.

Davenport starts this paper with the statement “The phrase 'Every flock has its black sheep' connotes the sporadic nature of their appearance. They crop out in flocks of breeding ewes and rams that are wholly white”. In a letter to Nature in 1880, Charles Darwin had also discussed the same “occasional appearance” of black or partly-black sheep in otherwise white flocks. Now, just a few years after the rediscovery of Mendelism, Davenport follows the above two opening sentences with a Mendelian hypothesis: “When a quality suddenly arises from parents that have its opposite the probability is that the two opposed qualities follow Mendel's law in inheritance and that the new, filial character is recessive, the parental opposite dominant.” He then tests this hypothesis with wool-colour and pedigree data of 877 sheep in the flock of Dr Alexander Graham Bell (1904). The results of four types of matings (described in terms of the Mendelian hypothesis) were:


White offspring

Black offspring

recessive homozygote (black) x recessive homozygote (black)



recessive homozygote (black) x heterozygote (white)



heterozygote (white with mixed ancestry) x heterozygote (white with mixed ancestry)



recessive homozygote x dominant homozygote (white with no known mixed ancestry)




Davenport was concerned that the proportion of black in the third type of mating was too low to be consistent with the Mendelian expectation of 25%. However, had he been aware of Pearson’s chi-squared test of independence (published only a few years earlier, in 1900), he would have been consoled to know that when the observed ratio of 40:7 is tested against the Mendelian expectation of 35.25:11.75, the resultant P value is 0.11, indicating consistency with the Mendelian hypothesis of 25% black.

Davenport’s justifiable final sentence was “The conclusion of the whole matter is that black wool color in sheep behaves like a Mendelian recessive characteristic.”

For further information on this trait, see OMIA 000201-9940: Coat colour, agouti in Ovis aries


Bell, A.G. (1904). Sheep Catalogue of Beinn Bhreagh, Victoria Co., Nova Scotia; showing the Origin of the Multinippled Sheep of Beinn Breagh and giving all the Descendants down to 1903, pp. 22. Washington. View this document

Darwin, C.R. (1880) Black sheep. Nature 23: 193. View this paper

Pearson, K. (1900). On the criterion that a given system of deviations from the probable in the case of a correlated system of variables is such that it can be reasonably supposed to have arisen from random sampling. Philosophical Magazine, 50: 157–175. View this paper

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