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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:

Dominant/recessive	Species	Further information
Normal/waltzing habit	mouse	http://www.informatics.jax.org/allele/MGI:1861234
Presence/absence of extra toe	chicken	OMIA 000810-9031: Polydactyly in Gallus gallus
Pea comb/single comb	chicken	OMIA 000782-9031: Pea comb in Gallus gallus
Rose comb/single comb	chicken	OMIA 000884-9031: Rose comb in Gallus gallus
White/yellow shanks	chicken	OMIA 001449-9031: Skin/shank colour, yellow in Gallus gallus
White/brown coloration	chicken	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”:

Mating	Polled offspring	Horned offspring
Polled Angus/Galloway/Red Polled x horned	104	13
First-cross x pure polled animal	23	1
First-cross x horned	24	18

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.

Conclusion

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.”

References

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