# Popular Science Monthly/Volume 81/October 1912/The Inheritance of Fecundity

Layout 4
* THE INHERITANCE OF FECUNDITY
By Dr. RAYMOND PEARL
MAINE AGRICULTURAL EXPERIMENT STATION
A THOROUGH and searching investigation of two great biological problems is a necessary prerequisite to any substantial advance of the science of eugenics. These problems are:
1. The mode of inheritance of human characters and traits of all kinds.
2. The physiology of reproduction in man, particularly with reference to human fecundity and fertility.
The progressive decline of the birth-rate in all, or nearly all, civilized countries is an obvious and impressive fact. Equally obvious and much more disturbing is the fact that this decline is differential. Generally it is true that those racial stocks which by common agreement are of high, if not the highest value, to the state or nation, are precisely the ones where the decline in reproduction rate has been most marked.
The causes concerned in the production of these results are without question exceedingly complex and difficult, if not impossible, of complete analysis. But of one thing we may be certain; somewhere in the complex of causes is included the biological factor as one element. Fecundity and fertility are physiological characters of the organism, subject to variation and capable of being inherited, just in the same manner as structural characters. We must be in possession of definite information regarding the physiology of fecundity and fertility, before it will be possible to make safe and sure advance in the social and eugenic analysis of matters involving these factors, such as, for example, the declining birth-rate.
The basic eugenic significance of that characteristic of organisms termed fecundity furnishes sufficient justification, I hope, for bringing to the attention of this Congress certain results regarding fecundity in one of the lower animals, namely the domestic fowl. In some particulars the results are, I believe, novel. They indicate, for the first time, the precise mode by which this complex physiological character fecundity is inherited. It will be the purpose of this paper to present—necessarily very briefly and without the detailed supporting evidence—the essential results of a study of fecundity in poultry, pointing out at the end some possible eugenic bearings of the results:
​During the course of this investigation into the inheritance of fecundity in the domestic fowl, which has now involved thirteen generations and several thousand individuals, two definite and clear-cut results have come to light. These are:
First: that the record of egg production or fecundity of a hen is not, of itself, a criterion of any value whatsoever from which to predict the probable egg production of her female progeny. An analysis of the records of production of large numbers of birds shows beyond any possibility of doubt that, in general, there is no correlation between the egg production of individuals and either their ancestors or their progeny.
Second: that, notwithstanding the fact just mentioned, fecundity is, in some manner or other, inherited in the domestic fowl. This must clearly be so, to mention but a single reason, because it has been possible to isolate and propagate from a mixed flock "pedigree lines" or strains of birds which breed true, generation after generation, to definite degrees of fecundity. Some of these lines breed true to a high condition or degree of the character fecundity; others to a low state or degree.
Definite as these results are, they give no clue as to how fecundity is inherited; what the mechanism is. It is believed that now a first approximation to the solution of this problem has finally been reached. While there remain obscure points yet to be cleared up, and more data are needed definitely to decide between certain alternatives, yet the results now in hand appear to indicate quite clearly the general character of the mechanism of the inheritance of fecundity, and to show what lines further investigation of the problem may most profitably take.
At the outstart it will be well to understand clearly what is meant by the term fecundity as here used. I have used the term "fecundity" only to designate the innate potential reproductive capacity of the individual organism, as denoted by its ability to form and separate from the body mature germ cells. Fecundity in the female will depend upon the production of ova and in the male upon the production of spermatozoa.
Fecundity is obviously a character depending upon the interaction of several factors. In the first place the number of ova separated from the body by a hen or any other animal must depend, in part at least, upon an anatomical basis, namely the number of ova present in the ovary and available for discharge. Further there must be involved a series of physiological factors. It has been possible to prove that the mere presence of an anatomically normal reproductive system, including a normal ovary with a full complement of ova, and a normal
​oviduct, is not enough to insure that a hen shall lay eggs, that is, exhibit actual as well as potential fecundity. While comparatively very rare, cases do occur in which a bird possesses a perfect ovary and perfect oviduct and is in all other respects entirely normal and healthy, yet never lays even a single egg in her life time. Such cases as these prove: first, that what we may call the anatomical factor is not alone sufficient to make potential fecundity actual; and second, that the anatomical and physiological factors are distinct, in the sense that the normal existence of one in an individual does not necessarily imply the coexistence of the other in the same individual.
Turning now to the physiological factors involved in fecundity, it would appear that there are at least two such factors or groups of factors. The first of the physiological factors involved may be designated the "normal ovulation" factor. By this is meant the complex of physiological conditions which, taken together, determine the laying of about such a number of eggs as represents the normal reproductive activity of the wild Gallus bankiva. It must be remembered that, for reasons which can not be gone into here, under conditions of domestication the activity of this normal ovulation factor will mean the production of considerably more eggs than under wild conditions. Egg production involves certain definite and rather severe metabolic demands, which under wild conditions will not always, or even often be met. Further, as has been especially emphasized by Herrick, egg-laying in wild birds is simply one phase of a cyclical process. If the cycle is not disturbed in any way the egg production is simply the minimum required for the perpetuation of the race. If, however, the cycle is disturbed, as, for example, by the eggs being removed from the nest as fast as they are laid, a very considerable increase in the total number of eggs produced will result.
It is a fact well known to poultrymen, and one capable of easy observation and confirmation, that different breeds and strains of poultry differ widely in their laying capacity. In saying this the writer would not be understood to affirm that a definite degree of fecundity is a fixed and unalterable characteristic by any particular breed. The history of breeds shows very clearly that certain breeds now notably poor in laying qualities were once particularly good. One of the best examples of this is the Polish fowl. But, in spite of this, not only do these breed and strain differences in fecundity exist, and probably always have existed, but they are inherited. Such inherited differences are independent of feeding or any other environmental factors. Thus a strain of Cornish Indian Games with which I have worked are poor layers, regardless of how they are fed and handled. This is merely a statement of particular fact; it does not imply that there may not exist other strains of Cornish Indian Games that are good layers.
​Now in individuals which are high layers, and have this characteristic in hereditary form, there must be involved some sort of physiological factor in addition to the normal ovulation factor already discussed. An analysis of extensive statistics has shown that high fecundity represents essentially an addition of two definite seasonal, laying cycles to the basis normal reproduction cycles. These added periods of productivity are what may be called the winter cycle and the summer cycle. The winter cycle is the more important of these. It is the best measure of relative fecundity which we have and has been used as the chief unit of fecundity in these studies. It constitutes a distinct and definite entity in fecundity curves. The existence of these added fecundity cycles in high laying birds must depend upon some additional physiological factor of mechanism besides that which suffices for the normal reproductive egg production. Given the basic anatomical and physiological factors, the bird only lays a large number of eggs if an additional factor is present.
We may next consider in greater detail these factors influencing fecundity, taking first
The Anatomical Basis of Fecundity
Since, as already pointed out, egg production obviously depends in part upon the presence of ova in a normal ovary, a question which demands consideration is the following:
To what extent are observed variations in fecundity (i. e., in the number of eggs laid) to be referred to anatomical differences? In other words, does the ovary of a high-producing hen with, for example, a winter record of from 75 to 115 eggs contain a larger number of oocytes than does the ovary of a hen which is a poor producer, laying no eggs in the winter period and perhaps but 10 or 15 eggs in the year?
To get light upon this question the observations to be described have been made. The object was to arrive at as accurate a relative judgment as possible regarding the number of oocytes in the ovaries of different individual birds. It is, of course, impossible practically to determine accurately the total absolute number of oocytes in the ovary. What can be done, however, is to count the number of oocytes which are visible to the unaided eye. While such results do not tell us, nor enable us to estimate with great accuracy, the total number of oocytes in the ovary, they do, nevertheless, throw interesting and useful light on the questions raised above. Some counts of this kind are shown in Table I.
From this table it is in the first place clear that the number of oocytes in the ovary of a hen is very large; much larger, I think, than has generally been supposed. While, to be sure, there are for the most part only vague statements respecting this point in the literature, ​usually these statements are to the effect that the bird's ovary contains "several hundred" ova.
Table I
Showing the Number of Visible Oocytes in the Ovary of Certain Fowls
| Case No. | Bird No. | Breed | Winter Pro- duction | Total Visible Oocytes |
| 1 | 8,021 | Barred Plymouth Rock | 3 | 1,228 |
| 2 | 8,017 | Barred Plymouth Rock | 0 | 1,666 |
| 3 | 8,030 | Barred Plymouth Rock | 0 | 914 |
| 4 | 8,005 | Barred Plymouth Rock | 5 | 1,174 |
| 5 | 1,376 | Barred Plymouth Rock | 3 | 2,306 |
| 6 | 8,018 | Barred Plymouth Rock | 0 | 1,194 |
| 7 | 8,009 | Barred Plymouth Rock | 0 | 2,101 |
| 8 | 8,010 | Barred Plymouth Rock | 5 | 1,576 |
| 9 | 425 | Barred Plymouth Rock | 0 | 1,521 |
| 10 | 3,546 | White Leghorn | 54 | 2,452 |
| 11 | 2,067 | White Leghorn | 32 | 3,605 |
| 12 | 3,453 | White Leghorn | 0 | 1,701 |
| 13 | 3,833 | White Leghorn | 0 | 2,145 |
| 14 | 52 | Cornish Indian Game | 13 | 1,550 |
| 15 | 71 | F$_1$ Cross | 106 | 2,000 |
Not only is the absolute number of oocytes large, but it is also very much larger than the number of eggs which any hen ever lays. A record of 200 eggs in the year is a high record of fecundity for the domestic fowl, though in exceptional cases it may go even a hundred eggs higher than this. But even a 200-egg record is only a little more than a tenth of the average total number of visible oocytes in a bird's ovary, to say nothing of the probably much larger number of oocytes invisible to the unaided eye, but capable of growth and development. In other words, it is quite evident from these figures that the potential "anatomical" fecundity is very much higher than the actually realized fecundity. This is true even if we suppose the bird to be allowed to live until it dies a natural death.
An examination of the table in detail indicates that there is no very close or definite relationship between the number of visible oocytes on the ovary and the winter production of a bird. Thus No. 1,367 and No. 3,546 each have about the same number of visible oocytes, yet one has a winter production record 18 times as great as the other. Again No. 71 with the extraordinarily high winter record of 106 eggs has only a little more than one half as many visible oocytes as hen No. 2,067, whose winter production record is only 32 eggs. Again, No. 71 with its 106 record has very nearly the same oocyte count as No. 8,010 with a winter record of zero. In general it may be said that the present figures give no indication that there is any correlation between fecundity as measured by winter production, and the number of oocytes in the ovary. Of course, the present statistics are meager. More ample figures are needed (and are being collected) from which to measure the correlation between actual and "anatomical" fecundity.
​The data now in hand, however, indicate clearly, it seems to me, that there must be some other factor than the anatomical one involved in the existence of different degrees of actual fecundity in the domestic fowl. It evidently is the case that when one bird has a winter record of twice what another bird has it is not because the first has twice as many oocytes in the ovary. On the contrary, it appears that all birds have an anatomical endowment entirely sufficient for a very high degree of fecundity, and in point of fact quite equal to that possessed by birds which actually accomplish a high degree of fecundity. Whether or not such high fecundity is actually realized evidently depends then upon the influence of additional factors beyond the anatomical basis. As has already been indicated in the preceding section, it is reasonable to suppose that these factors are physiological in nature.
The Mechanism of the Inheritance of Fecundity as Measured by Winter Egg Production
A study of numerous statistics shows that hens fall into three well defined classes in respect to winter production. These classes include (a) those birds which lay no eggs whatever in the winter period (up to March of the laying year); (b) those that lay but have a production during the period of something under about 30 eggs; and finally (c) those whose production exceeds 30 eggs in the winter period. The division point between the two latter classes is not sharply defined in every case, but it is plainly at about 30 eggs in the case of the breeds and strains used in these experiments. Since in the analysis some fixed point must be taken for this boundary, a production of 30 has been chosen for this purpose and will be used throughout. This is an arbitrary choice only in the sense that it is a convenient round number lying very near where the biological division point falls, at least in the strains of domestic fowls used in these experiments. The analysis could doubtless be carried through nearly or quite as well by taking the division point at a production of 29 or 31, but 30 is a more convenient figure.
In making the division of winter egg production into three groups it must be remembered that this is a character subject to purely somatic fluctuations and environmental influence. Allowance for these factors must be made in interpreting and classifying results.
Turning now to the symbolic analysis, we have to deal with three factors. These are:
1. An anatomical factor. This is basic. It consists in the presence of a normal ovary, the primary organ of the female sex. In the genetic analysis a separate letter need not be used for the designation of this factor, but instead it will be understood to be included in the letter denoting the presence of the female sex or its determiner. That is, $F$ ​will denote the presence of the ovary or the ♀ sex determiner. Then $f$ will denote the absence of femaleness and the absence of an ovary. Obviously a separate letter is not needed for this "anatomical factor," since the presence of an ovary is the objective criterion of the existence of the female sex, its absence of the existence of the male sex.
2. The "first production" factor. This is the primary physiological factor which in coexistence with $F$ makes the bird lay eggs during the winter period. Quantitatively it may be taken as determining a winter production of more than zero eggs and less than 30. The presence of this factor will be denoted by $L_{1}$.
3. The "second production" factor. This is a second physiological factor, which in coexistence with $F$ and $L_{1}$ leads to high fecundity. The presence of this factor will be denoted by $L_{2}$ and its absence by the corresponding small letter. When $F$ and $L_{1}$ are present the addition of $L_{2}$ makes a winter production of over 30 eggs. If $F$ is present and $L_{1}$ absent the presence of $L_{2}$ leads to a winter production of under 30 eggs. Thus either $L_{1}$ or $L_{2}$ alone makes a record of 30 eggs. They are independent determiners of this degree of production. It should be pointed out, however, that in spite of their equivalence in this regard the factors $L_{1}$ and $L_{2}$ are not qualitatively the same. That is, the increased production when $L_{1}$ and $L_{2}$ are both present is not because there are present two "doses" of the same determiner. The proof of this is found in the fact that when there are two "doses" of $L_{1}$ present in a bird it does not make her a high producer. $L_{2}$ may be considered an excess production factor, which erects a superstructure on the foundation furnished by $L_{1}$. In the absence of $L_{1}$ it lacks the foundation from which to start, and hence only can build about as high as $L_{1}$ would alone. Of course, it will be understood that in the presence of $f$ (absence of female sex and ovary) these physiological fecundity factors $L_{1}$ and $L_{2}$ are simply latent.
Using the letters in the manner defined above, and with the usual Mendelian method of writing gametic and zygotic formulæ, the data indicate that there exist 9 different types (in respect to fecundity) of Barred Plymouth Rock males, 6 types of Barred Plymouth Bock females, 3 types of Cornish Indian Game males, and 3 types of Cornish Indian Game females. The only point needing particular attention in reference to these formulas is that the factor $L_{2}$, the excess production factor, behaves in inheritance as a sex-limited or sex-correlated character. It is repelled by the female determiner $F$. It is thus like the barred pattern factor in the Barred Plymouth Rock fowl. 3 In consequence gametes of the type $FL_{2}$ are never formed. Any gamete which bears $F$ does not, under any circumstance, ever carry $L_{2}$. All females ​which carry the excess production factor $L_{2}$ are heterozygous in respect to it.
We have fecundity practically determined, then, by two physiological factors, one of which is sex-correlated in its inheritance and the ether not.
Table II
Showing Some Results of Mating Together Barred Plymouth Bock Males and Barred Plymouth Rock Females of Different Fecundity Genotypes. Summarized Data
The accordance between observed fact and theoretical expectation on this interpretation of the results is shown in the following tables ​which give the results of a portion of the actual experiments. As the experiments were rather extensive, it is not possible here to present anything like the complete material. Only representative matings are here given. Table II. gives the results of some of the B. P. R. × B. P. B. matings in detail, in order to show, not only the accordance between observation and theory, but also the distinctness of the classes of fecundity segregated (shown by the average winter production in each segregated class).
From the data set forth in the above table there can be no doubt as to the fact of the Mendelian segregation of fecundity, nor as to the entire distinctness of the things segregated.
In order to give a general survey of the results, and to demonstrate the reality of segregation over the wide range of material included in the experiments, the summary Table III is presented.
Table III
Showing the Observed and Expected Distribution in Respect of Fecundity of the Adult Female Offspring from all Matings in each of the Classes Tested in the Experiments
| Matings | Winter Production of Daughters | Winter Production of Daughters | Winter Production of Daughters | Winter Production of Daughters | Winter Production of Daughters | Winter Production of Daughters | Winter Production of Daughters |
| Matings | Class | Over 30 | Over 30 | Under 30 | Under 30 | Zero | Zero |
| All Barred Plymouth Rock × Barred | Observed | 365 | .5 | 259 | .5 | 31 | |
| Plymouth Rock. | Expected | 381 | .45 | 257 | .25 | 17 | .30 |
| All Cornish Indian Game × Indian | Observed | 2 | | 23 | | 15 | |
| Game. | Expected | 0 | | 25 | | 15 | |
| All F$_1$ (B.P.R. × C.I.G. and reciprocal | Observed | 36 | | 79 | | 8 | |
| cross). | Expected | 26 | .5 | 86 | .75 | 9 | .75 |
| All F$_2$ (F$_1$ × F$_1$, and F$_1$ × parent | Observed | 57 | .5 | 98 | .5 | 23 | |
| forms in all possible combinations). | Expected | 68 | .60 | 95 | .00 | 15 | .40 |
Considering the nature of the material and the character dealt with the agreement shown between observation and hypothesis is certainly as close as could reasonably be expected. Such discrepancies as are shown in the above table are fully discussed and their probable physiological explanations set forth in detail in the complete account of these experiments.
The detailed data given in the complete paper, of which the above discussion and tables give merely a very incomplete abstract, appear definitely to establish the following points:
1. That fecundity in the domestic fowl is inherited strictly in accordance with Mendelian principles.
2. That observed individual variations in fecundity here depend upon two separately inherited physiological factors, $L_{1}$ and $L_{2}$.
3. That high fecundity is manifested only when both of these factors are present together in the same individual.
4. That either of these factors when present alone whether in ​homozygous or heterozygous form causes about the same degree of low fecundity to be manifested.
5. That one of these factors, namely $L_{2}$, is sex-limited or sex-correlated in its inheritance, in such way that in gametogenesis any gamete which bears the female sex determinant F does not bear $L_{2}$.
6. That there is a definite and clear-cut segregation of high fecundity from low fecundity, in the manner set forth above.
These conclusions are fully and independently substantiated by long-continued breeding experiments involving the breeding together of (1) Barred Plymouth Rock males and females (a breed of generally high fecundity), (2) Cornish Indian Game males and females (a breed of generally low fecundity, (3) the $F_{1}$ and $F_{2}$ offspring from reciprocal crosses of Barred Plymouth Eocks and Cornish Indian Games and all possible matings inter se and with the parent forms of the cross-bred $F_{1}$ and $F_{2}$ offspring.
While these results may have no direct eugenic bearing, they do, I believe, have an important indirect connection with eugenic problems. In the first place, these results furnish a novel conception of the mode of inheritance of fecundity. They show that this highly variable physiological character is inherited in accord with simple Mendelian principles. They further show that simple selection of highly fecund females alone is not sufficient to ensure high fecundity in the race.
From the eugenic standpoint they suggest, though of course they do not prove, that possibly some part of the observed decline in human fecundity in highly civilized races may be due to the dropping out or loss of one or more of the genes upon which high fecundity depends, this loss being coincident with the complete cessation of the natural selection of highly fecund types.
Finally, these results on fecundity in fowls not only emphasize the importance of analytical studies to determine the precise mode of inheritance of human fecundity, but they also furnish a guide and stimulus for the conduct of such studies. If, as in the actual fact, it can be shown that in one animal belonging to the same great phylum to which man himself belongs (the vertebrate) fecundity is inherited in a simple Mendelian fashion, it encourages one to hope that some time a solution of the same problem may be reached for man. It at least points the way to a mode of attacking this complex problem which gives greater promise of leading ultimately to a solution than does any method which has hitherto been applied to it.
* ↑ This paper was read at the First International Eugenics Congress, held in London, July 24-30, 1912.
* ↑ The results set forth below were first presented at the meeting of the American Society of Naturalists at Princeton, N. J., in December, 1911. A complete report with full presentation of the experimental data will shortly be published in the Journal of Experimental Zoology.
* ↑ Cf . Pearl, R., and Surface, F. M., Arch. f. Entwick. Mech., Bd. XXX., pp. 45–61, 1910, and Science, N. S.; Vol. XXXII., pp. 870-874, 1910.
* ↑ The records of 1/2 refer to birds whose winter production record was exactly 30 eggs. Each one of the few birds of this sort is divided between the "Over 30" and the "Under 30" classes.