CHAPTER I.

THE SPLIT OR Y COMB.

A. INTERPRETATION OF THE Y COMB.

When a bird with a single comb, which may be conveniently symbolized as I, is crossed with a bird with a "V" comb such as is seen in the Polish race, and may be symbolized as oo, the product is a split or Y comb. This Y comb is a _new form_. As we do not expect new forms to appear in hybridization, the question arises, How is this Y comb to be interpreted? Three interpretations seem possible. According to one, the antagonistic characters (allelomorphs) are I comb and oo comb, and in the product neither is recessive, but both dominant. The result is a case of particulate inheritance--the single comb being inherited anteriorly and the oo comb posteriorly. On this interpretation the result is not at all Mendelian.

According to the second interpretation the hereditary units are not what appear on the surface, but each type of comb contains two factors, of which (in each case) one is positive and the other negative. In the case of the I comb the factors are presence of median element and absence of lateral or paired element; and in the case of the oo comb the factors are absence of median element and presence of lateral element. On this hypothesis the two positive factors are dominant and the two negative factors are recessive.

The third hypothesis is intermediate between the others. According to it the germ-cells of the single-combed bird contain a median unit character which is absent in the germ-cells of the Polish or Houdan fowl. This hypothesis supposes further that the absence of the median element is accompanied by a fluctuating quantity of lateral cere, the so-called V comb.

The split comb is obtained whenever the oo comb is crossed with a type containing the median element. Thus, the offspring of a oo comb and a pea comb is a split pea comb, and the offspring of a oo comb and a rose comb is a split rose. The three hypotheses may consequently be tested in three cases where a split comb is produced.

TABLE 1.

+-----------------------+-----+-----+------------+ | | I | Y | No median. | +-----------------------+-----+-----+------------+ | I × I | 100 | 0 | 0 | | I × Y | 50 | 50 | 0 | | I × no median | 0 | 100 | 0 | | Y × no median | 0 | 50 | 50 | | No median × no median | 0 | 0 | 100 | +-----------------------+-----+-----+------------+

The first and third hypotheses will give the same statistical result, namely, the products of two Y-combed individuals of F1 used as parents, will exhibit the following proportions: median element, 25 per cent; split comb, 50 per cent; and no median element, 25 per cent. These proportions will show themselves, whatever the generation to which the Y-combed parents belong, whether both are of generation F1, or F2, or F3, or one parent of one generation and the other of another. Other combinations of parental characters should give the proportions in the progeny shown in table 1.

On the second hypothesis, on the other hand, the proportions of the different kinds occurring in the progeny will vary with the generation of the parents. This hypothesis assumes the existence in each germ-cell of the original parent of two comb allelomorphs, _M_ and _l_ in single-combed birds and _m_ and _L_ in the Polish fowl, the capital letter standing for the presence of a character (Median element or Lateral element) and the small letter for the absence of that character. Consequently, after mating, the zygote of F1 contains all 4 factors, _MmLl_, and the soma has a Y comb; but in the germ-cells, which contain each only 2 unlike factors, these factors occur in the following 4 combinations, so that there are now 4 kinds of germ-cells instead of the 2 with which we started. These are _ML_, _Ml_, _mL_, and _ml_. Furthermore, since in promiscuous mating of birds these germ-cells unite in pairs in a wholly random fashion, 16 combinations are possible, giving 16 F2 zygotes (not all different) as shown in table 2.

TABLE 2.

+-------+---------------+---------+ | Type. | Zygotic | Soma. | | | constitution. | | +-------+---------------+---------+ | _a_ | M2L2[A] | Y | +-------+---------------+---------+ | _b_ | M2Ll | Y | +-------+---------------+---------+ | _c_ | MmL2 | Y | +-------+---------------+---------+ | _d_ | MmLl | Y | +-------+---------------+---------+ | _e_ | M2Ll | Y | +-------+---------------+---------+ | _f_ | M2l2 | I | +-------+---------------+---------+ | _g_ | MmLl | Y | +-------+---------------+---------+ | _h_ | Mml2 | I | +-------+---------------+---------+ | _i_ | mLML | Y | +-------+---------------+---------+ | _k_ | mLMl | Y | +-------+---------------+---------+ | _l_ | m2L2 | oo | +-------+---------------+---------+ | _m_ | m2Ll | oo | +-------+---------------+---------+ | _n_ | mlML | Y | +-------+---------------+---------+ | _o_ | mlMl | I | +-------+---------------+---------+ | _p_ | m2Ll | oo | +-------+---------------+---------+ | _q_ | m2l2 | Absent | +-------+---------------+---------+ [A] This convenient form of zygotic formulæ, using a subscript 2 instead of doubling the letter, is proposed by Prof. W. E. Castle.

It is a consequence of this second hypothesis that, in F2, of every 16 young 9 should have the Y comb; 3 the I comb; 3 the oo comb, and 1 no comb at all. It follows further that the progeny of two F2 parents will differ in different families. Thus if a Y-combed bird of type _a_ be mated with a bird of any type, _all_ of the progeny will have the Y comb.

From Y-combed parents of various types taken at random 4 kinds of families will arise having the following percentage distribution of the different types of comb:

1. Y comb, 100 per cent. 2. Y comb, 75 per cent; I comb, 25 per cent. 3. Y comb, 75 per cent; oo comb, 25 per cent. 4. Y comb, 56.25 per cent; I comb, 18.75 per cent; oo comb, 18.75 per cent; absent, 6.25 per cent.

Again, mating two extracted I combs of F2 should yield, in F3, two types of families in equal frequency as follows:

1. I comb, 100 per cent. 2. I comb, 75 per cent; no comb, 25 per cent.

Again, mating two extracted oo combs of F2 should yield, in F3, two types of families in equal frequency, as follows:

1. oo comb, 100 per cent. 2. oo comb, 75 per cent; no comb, 25 per cent.

Single comb × Y comb should give families of the types:

1. Y comb, 100 per cent. 2. Y comb, 50 per cent; I comb, 50 per cent. 3. Y comb, 50 per cent; oo comb, 50 per cent. 4. Y comb, 25 per cent; I comb, 25 per cent; oo comb, 25 per cent; absent, 25 per cent.

Mating oo comb and Y comb should give the family types:

1. Y comb, 100 per cent. 2. Y comb, 50 per cent; oo comb, 50 per cent. 3. Y comb, 50 per cent; I comb, 50 per cent. 4. Y comb, 25 per cent; oo comb, 25 per cent; I comb, 25 per cent; no comb, 25 per cent.

Finally, I comb and oo comb should give the following types of families:

1. Y comb, 100 per cent. 2. I comb, 100 per cent. 3. Y comb, 50 per cent; oo comb, 50 per cent. 4. I comb, 50 per cent; no comb, 50 per cent.

Now, what do the facts say as to the relative value of these three hypotheses? Abundant statistics give a clear answer. In the first place, the progeny of two Y-combed F1 parents is found to show the following distribution of comb types: Y comb 471, or 47.3 per cent; I comb 289, or 29.0 per cent; oo comb 226, or 22.7 per cent; and no comb 10, or 1 per cent. The presence of no comb in F2 speaks for the second hypothesis, but instead of the 6.25 per cent combless expected on that hypothesis only 1 per cent appears. There is no close accord with expectation on the second hypothesis.

Coming now to the F3 progeny of two Y-combed parents, we get the distribution of families shown in table 3.

TABLE 3.

+--------+-------------------------+---------------------------+ | Pen No.| Parents. | Comb in offspring. | | |-------------------------+------+-----+-----+--------| | | ♀ (F2) | ♂ (F2) | I | Y | oo | Absent.| |--------+-------------+-----------+------+-----+-----+--------+ | 707 | { 366 | 1378 | 18 | 16 | 9 | .... | | | { 522 | 1378 | 1 | 1 | 0 | .... | | | | | | | | | | | { 2250 | 2247 | 9 | 5 | 4 | 1 | | 763 | { 2700 | 2247 | 3 | 5 | 3 | 1 | | | { 3799 | 2247 | 5 | 4 | 3 | .... | | | | | | | | | | 769 | { 1305 | 911 | 7 | 4 | 6 | .... | | | { 2254 | 911 | 15 | 15 | 7 | .... | | |------+-----+-----+--------+ | Totals (142) | 58 | 50 | 32 | 2 | | Proportions (per cent) | 40.8 | 35.2| 22.5| 1.4 | | | | | ⎿⎵⎵⎵⏌ | | | | | 23.9 | +---------+-------------+----------+------+-----+--------------+

An examination of these families shows not one composed exclusively of Y-combed individuals nor those (of significant size) containing Y-combed and I-combed or oo-combed individuals exclusively, much less in the precise proportion of 3:1, yet such should be the commonest families if the second hypothesis were true. Notwithstanding the marked deviation--to be discussed later--from the expected proportions of I, 25 per cent; Y, 50 per cent; oo, 25 per cent, the result accords better with the first or third hypothesis. Since on either of these hypotheses the same proportions of the various types of comb are to be expected in the progeny of Y-combed parents of whatever generation, it is worth recording that from such parents belonging to all generations except the first the results given in table 4 were obtained, and it will be noticed that these results approach expectation on the first or third hypothesis.

TABLE 4.

+-----------+-------+-------+-------+---------+--------+ | | I | Y | oo | Absent. | Total. | +-----------+-------+-------+-------+---------+--------+ |Frequency | 235 | 291 | 144 | 12 | 682 | | | | | | | | |Percentage | 34.5 | 42.7 | 21.1 | 1.8 | .... | +-----------+-------+-------+-------+---------+--------+

The progeny of two extracted single-combed parents of the F2 generation give in 3 families the following totals: Of 95 F3 offspring, 94 have single combs; one was recorded from an unhatched chick as having a _slightly_ split comb, but this was probably a single comb with a slight side-spur, a form that is associated with purely I-combed germ-cells. This result is in perfect accord with the second and third hypotheses, but is irreconcilable with the first hypothesis.

The progeny of two extracted oo-combed parents is given in table 5.

TABLE 5.

+-----+-----------------+----------------------+ | | Parents. | Comb in offspring. | | Pen +--------+--------+----+----+----+-------+ | No. | ♀ | ♂ | I | Y | oo |Absent.| | | (F2). | (F2). | | | | | +-----+--------+--------+----+----+----+-------+ | 729 |{2255 | 936 | .. |[A]4| 36| .. | | |{2269 | 936 | .. | .. | 29| .. | | | | | | | | | | |{ 369 | 1390 | 1 | .. | 3| .. | | 756 |{1067 | 1390 | .. | .. | 8| 1 | | |{1113 | 1390 | .. | .. | 13| 4 | | | | | | | | .. | | |{2011 | 444 | .. | .. | 10| .. | | |{2011 | 2621 | .. | .. | 9| .. | | |{2333 | 444 | .. |[A]5| 11| .. | | |{2333 | 2621 | .. |[A]1| 2| .. | | 762 |{2618 | 444 | .. | .. | 2| .. | | |{2618 | 2621 | .. | .. | 5| .. | | |{3776 | 444 | .. | .. | 2| .. | | |{3776 | 2621 | .. | 1 | 14| .. | | | | | | | | | | |{2016 | 4731 | .. | .. | 10| .. | | 820 |{2255 | 4731 | .. | .. | 16| .. | | |{5143 | 4731 | .. | .. | 45| .. | | |{6479 | 4731 | .. | .. | 31| .. | | | | | | | | | | |{[B]2618| 5119 |[B]1| .. | 23| .. | | |{3776 | 5119 | .. | .. | 28| .. | | 832 |{4404 | 5119 | .. | .. | 9| .. | | |{4732 | 5119 | .. | .. | 3| .. | | |{5803 | 5119 | .. | .. | 21| 2 | | |{6481 | 5119 | .. | .. | 11| .. | | | | | | | | | | 834 | 2324 | 5090 | .. | .. | 26| .. | | +----+----+---+--------+ | Total | 2 | 11 | 367| 7 | +--------------+--------+----+----+------------+ [A] Median element recorded as "small" in these offspring. [B] A median element visible in the mother, No. 2618.

The distribution of offspring in the 24 families of table 5 is in fair accord with any of the three hypotheses, but seems to favor the second, for that hypothesis calls for families with combless children, whereas such are not to be expected on the first hypothesis. Moreover, agreement with the second hypothesis is fairly close, for that calls for 3 families with combless children and there were actually 3 such families showing a total of 1.8 per cent combless, where expectation is 2.8 per cent. What is opposed to any hypothesis is the appearance of some Y-combed offspring; and to account for this the hypothesis is suggested that the germ-cells of some parents with oo comb contain traces of the I-comb determiner. The word "traces" is used because the median element in these Y-combed offspring is practically always very small. It is fair, consequently, to conclude that oo × oo gives oo-combed, and occasionally combless, offspring. This conclusion is further supported by the statistics derived from extracted oo comb of _all_ generations bred _inter se_, which give: Y 11, oo 427, and no comb 8, where the 11 Y-combed birds are those just referred to as progeny of F2 parents. The non-median comb, consequently, probably contains only non-median germ-cells.

TABLE 6.

+---+-------------------------------------------------+--------------+ | | Parents. | Offspring. | | |--------+-----+----------+------+-----+----------+----+----+----+ |Pen| ♀ |Form | Degree | ♂ |Form | Degree | | | | |No.| (F2). | of | of | (F2).| of | of | I | Y | oo | | | |comb.|splitting.| |comb.|splitting.| | | | |---|--------|-----|----------|------|-----|----------|----|----|----| | | | | _P. ct._ | | | _P. ct._ | | | | | | { 427 | Y | 5 | 439 | I | 0 | 5 | 1 | .. | |628| { 722 | Y | 20 | 439 | I | 0 | 1 | 5 | .. | | | { 725 | Y | 10 | 439 | I | 0 | 5 | 3 | .. | | | | | | | | | | | | |629| 427 | I | 0 | 491 | Y | 50 | 9 | 6 | .. | | | | | | | | | | | | |765| 1790 | I | 0 | 1794 | Y | 90 | 17 | 25 | .. | | | | | | | | | | | | | | {3846 | I | 0 | 6652 | Y | 90 | 8 | 5 | .. | |802| {5025 | I | 0 | 6652 | Y | 90 | 14 | 11 | 2 | | | {5087 | I | 0 | 6652 | Y | 90 | 13 | 17 | 2 | | | | | | | | | | | | |812| {4254 | I | 0 | 4118 | Y | 90 | 15 | 13 | .. | | | {5540 | I | 0 | 4118 | Y | 90 | 8 | 9 | .. | | |----|----|----| | Totals (189) | 95 | 95 | 4 | | Percentages |49.0|49.0| 2.0| +-----------------------------------------------------+----+----+----+

The mating of extracted I comb and Y comb, both of the second (or later) hybrid generation, gives the following distribution of types in the offspring (table 6): Y comb 95 (49 per cent); I comb 95 (49 per cent); oo comb 4 (2 per cent). In detail the results given in table 6 accord badly with the second hypothesis, which demands some families with 100 per cent Y comb.

The mating of extracted oo comb×Y comb, where both parents are of the second hybrid generation, gave the distribution of comb types in the 6 families that are recorded in table 7.

TABLE 7.

+---+-----------------+-------------------------+ | | Parents. | Offspring. | |Pen|-----------------+-----+-----+-----+-------| |No.| ♀ | ♂ | I | Y | oo |Absent.| | | (F2). | (F2). | | | | | +------------+--------|-----+-----|-----|-------| |634| { 298 | 444 | 0 | 15 | 18 | .... | | | { 366 | 444 | 5 | 23 | 15 | .... | | | | | | | | | |729| { 913 | 936 | 2 | 28 | 37 | .... | | | { 935 | 936 | ....| 13 | 39 | .... | | | | | | | | | |756| { 1043 | 1390 | ....| 13 | 11 | 1 | | | { 1048 | 1390 | ....| 0 | 5 | .... | | |-----+-----+-----+-------+ | Totals (214) | 7 | 92 | 115 | 1 | +---------------------+-----+-----+-----+-------+

The single comb recorded in the case of 7 birds is doubtless merely the limiting condition of a Y comb in which the median element is developed to its fullest extent. All but 2 of the 7 were recorded from early embryos when an incipient bifurcation would be more difficult to detect. This explanation applies generally, and accounts for the usual excess of I comb when compared with Y comb, as for instance in table 3, page 7. Returning to table 7, it is, consequently, probable that only the Y-combed and non-median-combed offspring are produced and that they are in the proportion of 99 to 115 or of 46 per cent to 54 per cent. If we add together all records of a oo×Y cross, disregarding the generation of the parents, we get a total I comb 5,[1] Y comb 177, oo comb 172, and absent 3, or 182 (51 per cent) with the median element and 175 (49 per cent) without. Thus the oo×Y cross gives the 1:1 proportion called for on the first and third hypotheses and not at all the variety required by the second hypothesis.

[1] Excluding 6 doubtful because from too young embryos and not observed by myself.

TABLE 8.

+---+----------------------+---------------+------------------+ | | Mother. | Father. |Comb in offspring.| |Pen+---------+-----+------+--------+------+---+---+----+-----+ |No.| | |P. ct.| | | | | | | | | No. |Comb.|split.| No. |Comb. | I | Y | oo | Abs.| +---+---------+-----+------+--------+------+---+---+----+-----+ |704| { 65 F1| Y | 50 | 1420 F2|Absent|.. | 10| 6 | 8 | | | {1061 F2| Y | 50 | 1420 F2| Do. |.. | 4| .. | 1 | | | | | | | | | | | | |819| { 57 F1| Y | 50 | 1420 F2| Do. |.. | 8| 6 | 5 | | | { 65 F1| Y | 60 | 1420 F2| Do. |.. | 1| .. | 1 | | +---+---+----+-----+ | Total | 0 | 23| 12 | 15 | +------------------------------------------+---+---+----+-----+

Finally, we must consider the result of mating a bird without papillæ (No. 1420, pen 704) with a median-combed hen (480). When this typical single-combed hen was used the 49 progeny were all of the Y type.[2] This proves that the combless type behaves only as an extreme of the non-median type.

[2] One is reported as having a I comb; probably the limiting condition, again.

When Y-combed hens were used with the combless cock the offspring had Y comb and non-median-comb in nearly equal numbers, 23:27 (table 8), but the latter included an unusually large proportion of combless fowl (15 in 27). When a combless hen (No. 4257) was used, 9 of the offspring had oo comb and 2 no comb; not a greater proportion of combless birds than in the no-comb×Y-combed cross. All of these facts indicate that "comblessness" is not entire absence of the comb factors, but a minimum case of the oo or paired comb. This result is opposed to the second hypothesis.

The statistics of all matings between I, Y, and no comb on the one side and no comb on the other thus speak unanimously for the conclusion that in these matings we are not dealing with 2 pairs of allelomorphs, but with a single comb and its absence (third hypothesis) or with a case of particulate inheritance (first hypothesis). Moreover, it must be said that the split comb is obtained also when the Polish-Houdan comb is crossed with a pea comb or a rose comb; and the pea and rose combs can not be said to have "lateral comb absent," as required by the second hypothesis. Consequently the second hypothesis is definitely excluded.

It now remains to decide between the two remaining hypotheses. First of all, it may be said that the perfection with which I and oo combs can be extracted from Y-combed birds indicates that we are here dealing with a case of Mendelian inheritance and, in so far, favors the third hypothesis. To accord with the theory of particulate inheritance, of which the first hypothesis is a special case, the two united characters should transmit the mosaic purely; but this they do not do. Hence the third hypothesis is to be preferred to the first.

Comblessness is a necessary consequence of the second hypothesis and is inexplicable on the first hypothesis. On the third hypothesis it may be accounted for as follows: Absence of single comb is allelomorphic to its presence. The lateral comb is a character common to fowl either with or without the median comb, but it is ordinarily repressed in the birds with single comb and gains a large size when the median element is absent. It is a very variable element. At one extreme it forms the cup comb; at the other there is an absence of any trace of comb. My own records show all grades between these extremes, including minute papillæ on both sides of the head or on one side only, low paired ridges, the butterfly comb, and cup comb shorter than normal. This variability of the lateral element is comparable to the fluctuation in size of the single comb itself, as illustrated by the Single-comb Minorca on the one hand and the Cochin on the other. It is comparable, also, to the fluctuation in the paired part of the Y comb, which we shall consider in the next section, and to the variability of the oo comb as met with in the pens of fanciers.

The foregoing considerations do not, at first sight, account for the Y comb as seen in F1. Yet they provide us with all the data for an explanation. Median comb of the Minorca dominates over no median of the Polish, and so in F1 we have the median element represented. But, on the well-known principle of imperfection of dominance in F1, the median comb is usually incomplete and, probably for some ontogenetic reason, incomplete only behind. The incompleteness behind permits the development there of the elsewhere repressed lateral comb, and we therefore have the Y comb--evidence at the same time of a repressed lateral-comb Anlage in the single-combed birds and of imperfection of dominance of the single comb in the first hybrid generation.

B. VARIABILITY OF THE Y COMB AND INHERITANCE OF THE VARIATIONS.

As already stated, the proportions of the median and the lateral elements in the Y comb are very variable; the median element may, indeed, constitute anywhere from 100 per cent to 0 per cent of the entire comb. Even full brothers and sisters show this variability. Thus the offspring of No. 13 ♀ Single-comb Minorca and No. 3 ♂ Polish have the median element of the Y comb ranging from 0 per cent to 70 per cent of the whole comb. Notwithstanding this variability of the median element in any family there is a difference in the average and the range of variability in families where different races are employed. Thus the offspring of two Polish × Minorca crosses show an _average_ of 46 per cent of the median element in the comb; the Houdan × Minorca cross gives combs with 60 per cent of the median element; and in the combs of the offspring of two Houdan × White Leghorn crosses there is, on the average, 71 per cent of the median element. The Houdan × Dark Brahma (pea comb) gives combs with an average of 87 per cent median element and the Polish × Rose-comb Minorca cross gives 89 per cent median. The rose-combed hens used in this last cross were heterozygous, having single comb recessive; consequently they produced also chicks with typical Y combs. Such had, on the average, only 59 per cent of the median element and were thus in striking contrast with the slightly split rose combs. In the case of the partially split rose combs the median element ranged from 60 per cent to 100 per cent of the whole length of the comb; but in the split single combs the range is from 0 to 100 per cent. Thus, in the two cases, the proportion of the median element and the range of its variability differ greatly.

Also, in generations subsequent to the first, the Y comb exhibits this same variability. We have already seen that the progeny of the Y-combed offspring of any generation may be compared with those of any other, and so we may mass together the progeny of all hybrid generations so long as they are derived from the same ancestral pure races.

In inquiring into the meaning of this variability we must first construct the polygon of frequency of the various grades of median element. This is plotted in fig. A, which is a composite whose elements are, however, quite like the total curve. There is one empirical mode at 70 per cent and another at 0 per cent. The smaller mode at 50 per cent is, I suspect, due to the tendency to estimate in round numbers, and may be, in this discussion, neglected. From this polygon we draw the conclusions, first, that the median element in the Y comb tends to dominate strongly over the absence of this element, as 7:3, and, second, that dominance is rarely complete. Yet there is an important number of cases, even in F1, where the median element is almost or completely repressed (down to 10 to 0 per cent of the whole) and the comb consists of two high and long lateral elements--the "cup comb" of Darwin. There are, then, in the offspring of a median-combed and a non-median-combed parent, two types with few intergrades--the type of slightly incomplete dominance of the median element and the type of very incomplete dominance.

We have now to consider how these two types of comb and their fluctuations behave in heredity. When two parents having each combs of the 70 per cent or 80 per cent median type are mated, their offspring belong to the three categories of I, Y, and "no-median" comb, but the relative frequency of these three categories is not close to the ideal of 25 per cent, 50 per cent, and 25 per cent, respectively. For there is actually in 336 offspring a marked excess of the I comb, 36 per cent, 44 per cent, and 20 per cent, respectively, resulting. When, on the other hand, two parents having each combs of the 10 per cent and 0 per cent types are mated their offspring are of the same three categories and the proportions actually found in 241 offspring (28 per cent, 47 per cent, 25 per cent) closely approximate the ideal. It is clear, then, that even the cup comb, without visible median element, has such an element in its germ-cells and is totally different in its hereditary behavior from the Polish comb, in which the median element is absent, not only from the soma, but also from the germ-cells.

We have seen in the last paragraph that the Y comb with only 10 per cent to 0 per cent median element has germ-cells bearing median comb as truly as the Y comb containing 70 per cent to 80 per cent median element, but we have also seen that in the latter case there is an excess of single-combed progeny. We have now to inquire whether, in general, there is a close relation between the proportion of median element in the comb of the parents and the percentage of single-combed offspring. These relations are brought out in the lower half of table 9.

TABLE 9.--_Frequency of the different proportions of single element in the combs of offspring of parents having the average proportion of median element given in the column at the left._

+-------------+--------------------------------------------------------+ | | Y combs. | | +--------------------------------------------------------+ | | Offspring. | | +----+----+----+----+----+----+----+----+----+----+------+ | | 0 | 10 | 20 | 30 | 40 | 50 | 60 | 70 | 80 | 90 |Total.| +-------------+----+----+----+----+----+----+----+----+----+----+------+ | { 0 | 21 | 5 | 4 | 3 | 4 | 6 | 5 | 10 | 8 | 1 | 67 | | {10 | 21 | 5 | 3 | 0 | 3 | 9 | 2 | 4 | 2 | 0 | 49 | | {20 | 5 | 4 | 2 | 1 | 0 | 4 | 2 | 12 | 0 | 1 | 31 | | {30 | 8 | 17 | 8 | 10 | 9 | 22 | 12 | 30 | 8 | 3 | 127 | | Parents {40 | 9 | 7 | 4 | 2 | 7 | 39 | 18 | 46 | 26 | 5 | 163 | | {50 | 7 | 5 | 2 | 1 | 5 | 32 | 13 | 48 | 35 | 11 | 159 | | {60 | 10 | 7 | 2 | 2 | 2 | 19 | 14 | 47 | 51 | 15 | 169 | | {70 | 9 | 2 | 4 | 0 | 1 | 6 | 7 | 28 | 41 | 11 | 109 | | {80 | .. | .. | 1 | 1 | 1 | 1 | 6 | 12 | 11 | 6 | 39 | | {90 | .. | 2 | 1 | 0 | 0 | 3 | 0 | 3 | 8 | 9 | 26 | | +----+----+----+----+----|----+----+----+----+----+------+ | Total | 90 | 54 | 31 | 20 | 32 |141 | 79 |240 |190 | 62 | 939 | +======================================================================+ | | All types of combs in offspring. | | +----------+---------------------------------------------+ | |Number | I | Y | Non-median. | | |of +------+--------+------+--------+-----+-------+ | |offspring.| No. | P. ct. | No. | P. ct. | No. | P. ct.| |-------------+----------+------+--------+------+--------+-----+-------+ | { 0 | 146 | 42 | 20 | 67 | 46 | 37 | 25 | | {10 | 99 | 25 | 25 | 49 | 50 | 25 | 25 | | {20 | 73 | 22 | 30 | 31 | 43 | 20 | 27 | | {30 | 249 | 61 | 25 | 127 | 51 | 61 | 24 | | Parents {40 | 309 | 73 | 24 | 163 | 53 | 73 | 23 | | {50 | 329 | 93 | 28 | 159 | 48 | 77 | 23 | | {60 | 368 | 120 | 33 | 169 | 46 | 79 | 21 | | {70 | 232 | 80 | 35 | 109 | 47 | 43 | 18 | | {80 | 104 | 42 | 40 | 39 | 38 | 23 | 22 | | {90 | 75 | 38 | 51 | 26 | 34 | 11 | 15 | | +----------+------+--------+------+--------+-----+-------+ | Total | 1984 | 596 | 30.0 | 939 | 47.3 | 449 | 22.7| +-------------+----------+------+--------+------+--------+-----+-------+

The proportion of single-combed offspring in the total filial population is 30.0 per cent, a departure of such magnitude from the expected 25 per cent as to arrest our attention. Further inspection of table 9 shows that the excess of single-combed offspring is found only in the lower half of the series. When the percentage of median element in the parents is under 50 the proportions of I, Y, and no-median combs are as 25.5 per cent, 49.8 per cent, 24.7 per cent, or close to expectation; but when the percentage is 50 or over the proportions are, on the average, 33.6 per cent, 45.2 per cent, and 21.2 per cent, a wide departure from expectation, 1108 individuals being involved. An examination of table 9 shows, moreover, that the proportion of offspring with single comb rises steadily as the proportion of the median element in the parentage increases from 50 per cent. The meaning of this fact is at present obscure, but the suspicion is awakened that, while the "cup comb" and the more deeply split combs are typical heterozygotes the slightly split combs are a complex of 2 or more units, one of which is "single comb." But that this is not the explanation follows for two reasons: first, that even in the F1 generation slightly split combs are obtained, and, second, that the offspring of the cup combs are much more variable than those of slightly split combs (70 to 90 per cent median). What is strikingly true is that, from 50 per cent up, as the proportion of the median element in the parents increases the percentage of single-combed offspring rises.

The matter may be looked at in another light. Median comb is dominant over its absence. Typically, we should expect F1 to show a single comb; the Y comb that we actually get is a heterozygous condition due to the failure of the median comb to dominate completely. Typically we should expect F2 to reveal 75 per cent single combs, of which 1 in 3 is homozygous and 2 in 3 are heterozygous. Owing to the failure of single comb always to dominate completely in the heterozygotes, we expect to find some of the 75 per cent with the Y comb. When in the parents dominance has been very incomplete in the heterozygote (as is the case in the 0 per cent to 40 per cent median-combed parents) we find it so in the offspring also and all heterozygotes show a Y comb of some type. But when in the parents dominance has been strong in the heterozygote (50 per cent to 90 per cent) it is so in the offspring also and only a part of the heterozygotes show the Y comb; the others show the single comb and thus swell the numbers of the single-combed type. The only objection to this explanation is found in the reduction in the percentages of the no-median type. Thus, adding together the homozygous and heterozygous median-combed offspring and comparing with the non-median-combed, we find these ratios:

Parental per cent 0-40 50 60 70 80 90 Ratio 75.3:24.7 76:23 79:21 82:18 78:22 85:15

There is a great deviation from 25 per cent in the "non-median" offspring of the 90 per cent parents, but in this particular case the total number of offspring is not large, and the deviation has a greater chance of being accidental. Altogether this explanation of the varying per cents of single comb on the ground of inheritance of varying potency in dominance seems best to fit the facts of the case.

From the foregoing facts and considerations we may conclude that the Y comb represents imperfect dominance of median over no-median comb; that there is a fluctuation in the potency of the dominance, so that the proportion of the median element varies from 0 to over 90 per cent; that the more potent the dominance of median element is in any parents the more complete will be the dominance in the offspring and the smaller will be the percentage of imperfectly dominant, or Y-combed, offspring. _Dominance varies quantitatively and the degree of dominance is inheritable._

The index of heredity may be readily obtained in the familiar biometric fashion from table 9. This I have calculated and found to be 0.301± 0.002. This agrees with Pearson's theoretical coefficient of correlation between offspring and parent. The index is larger than it would otherwise be because it is measured with an _average_ of the parents and these parents assortatively mated. But this instance is, in any case, an interesting example of strong inheritance of a quantitative variation.

What, it may be asked, is the relation of these facts to the general principle that inheritance is through the gametes? Why, when a gamete with the median element unites with a gamete without that element, does the zygote develop a soma that in some cases shows a nine-tenths median and sometimes a one-tenth median element? We have seen that the Y comb is a heterozygous form due to imperfection of dominance of the median element; but why this variation in the perfection of the median element? This is probably a piece of the question, why any dominance at all. We find, in general, that the determiner of a well-developed organ dominates in the zygote over the determiner of a slightly developed condition of that organ or its obsolete condition. It is as though there were in the zygote an interaction between the strong and the weak form of the determiner, and the strong won; but sometimes the victory is imperfect. In the specific case of comb the interaction between median and no-median leads to a modification, weakening, or imperfection of the median element, and this weakening varies in degree. Sometimes the weakening is inappreciable--when the comb is essentially single; sometimes it is great, and the result is a comb in which the median element is reduced to one-half; sometimes, finally, the determiner of median comb is so completely weakened by its dilution with "no-median" as not to be able to develop, and we have the cup comb with only a trace of the median element. Nevertheless, such a cup comb is heterozygous and produces both single-combed and Polish-combed germ-cells. Thus the variation in the extent of the median comb seems to point to variations in relative potency of the median comb over its absence.