Table 44 shows clearly, first, that there are families of two booted parents that never fail to produce booted offspring. There is, however, even in pure-bred booted races, a marked variability in the grade of booting, extending from 3 (or 4) to 10. The significance of this variability must be left for future investigations. There is in the least boot, as it were, an extension of the field of activity of the feather-inhibiting factor that is always present on the hinder aspect of the shank, so that it interferes with the development of feathers on the inner face of the shank also.
In the first hybrid generation all somatic cells are hybrid. The feather inhibitor is present in the skin of the shank, but its strength is diluted by the presence in the same cells of a protoplasm devoid of the inhibiting property. Consequently, the prevailing grade of the boot falls from 6 (or 10) to 3. Despite the dilution, inhibition is complete in about 8 per cent of the offspring (grade 0); in about 10 per cent of the offspring the inhibiting factor is so weak that the boot develops as in the pure-blooded Brahma. When, as a result of inbreeding F1's, the feather-inhibiting factor is eliminated from certain offspring, and such full-feathered birds are bred together, we find a return of the mode to high numbers, such as 8 to 10 (but also 5). There is no doubt of segregation.
Table 44.—Brahma crosses. (All entries are percentages.)
| Percentage. | From table. | Boot-grade in offspring. | |||||||||||
| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | Average grade. | ||
| Pure blood | 31, B | ... | ... | ... | 3.3 | 3.3 | 6.6 | 24.6 | 4.9 | 9.8 | 14.8 | 32.8 | 7.62 |
| F1 (D × R) | 32 | 7.9 | 13.8 | 16.8 | 31.0 | 17.5 | 7.8 | 3.4 | 1.1 | 0.7 | ... | ... | 2.84 |
| Extracted R × R | 39 | ... | 0.3 | 0.7 | 4.2 | 7.7 | 13.6 | 10.5 | 9.8 | 18.5 | 16.0 | 18.8 | 7.25 |
| DR × RR | 40 | 2.3 | 2.7 | 9.8 | 15.1 | 16.7 | 14.3 | 11.9 | 7.3 | 8.8 | 7.2 | 4.0 | 5.04 |
| 50 p. ct. DR. | 50 p. ct. RR. | ||||||||||||
| DR × DR | 42 | 12.3 | 7.2 | 12.7 | 20.9 | 14.9 | 12.8 | 7.3 | 4.4 | 3.8 | 2.3 | 1.4 | 3.59 |
| 25 p. ct. DD. | 50 p. ct. DR. | 25 p. ct. RR. | |||||||||||
| DR × DD | 41 | 29.5 | 21.3 | 16.4 | 23.0 | 8.2 | 1.6 | ... | ... | ... | ... | ... | 1.69 |
| 50 p. ct. DD. | 50 p. ct. DR. | ||||||||||||
If a heterozygous bird be mated to a recessive the variability of the offspring is much increased, owing to the occurrence in the progeny of both DR and RR individuals (table 40). The offspring do not, to be sure, fall into two distinct and well-defined types, as in typical Mendelian cases; but one part of the range of variation agrees fairly with that of pure RR's, i. e., Brahmas, and the remainder with that of heterozygotes. And if we make the division in the middle of the middle class, viz, 5, we shall find a close approximation to that equality of extracted recessives and heterozygotes that the segregation theory calls for (table 44).
If, again, two heterozygous birds be mated, the variability is still greater and the proportion of clean-footed offspring rises to 12 per cent. These, together with some of the extremely slightly booted offspring, represent the extracted dominants. The whole range now falls into three regions divided by the middle of grades 2 and 5. These regions correspond to the DD's, the DR's, and the RR's of typical cases of segregation, and their relative proportions are approximately as 25: 50: 25.
Finally, if a heterozygote be mated to an extracted dominant the proportion of clean-footed offspring rises to about 30 per cent and the whole range of variation falls readily into two parts, the one comprising grades 0 and 1, the other grades 2 and above. The first includes the DD offspring; the second, the DR's; and their frequency is equal. One will not fail to note that we are not here dealing with a case of blending simply, and the inheritance of the blend; such a view is negatived by the fact of the much greater variability of DR × DR cross over the simple D × R cross of the first generation. One may safely conclude, then, that, despite the apparent blending of booting characters in the first generation of hybrids, true segregation takes place. But this is always to be seen through the veil of imperfect dominance.
A casual examination of table 38 would seem to show a correlation between the grade of booting of the parents and that of the average of their progeny. Thus, on the whole, the parental grades run high in the upper part of the table and run low in the lower part. This relation would thus seem to confirm Castle's conclusion for polydactylism in guinea-pigs that there is an inheritance of the degree of a character. One consequence of such an inheritance would be that it would be possible in a few generations to increase or diminish the grade of a character and fix any required grade in the germ-plasm. A more careful consideration of the facts of the case shows that this relation has another interpretation. The grade of boot of the different parents varies largely because their gametic constitution is diverse. As table 39 shows, the parents of the upper part of table 38 are chiefly extracted recessives, and consequently their booting and that of their offspring are characterized by high grades. On the other hand, the parents of the lower part of the table are heterozygous or extracted dominants and, consequently, their grades and also those of their offspring average low. On account of the lack of homogeneity of the families in table 38, one can draw from it no proper conclusions as to relation between parental and filial grades. On the other hand, from a homogeneous table, like table 39, we can hope to reach a conclusion as to the existence of such a relation. I have calculated, in the usual biometric fashion, the coefficient of correlation between average parental and filial grades, and found it to be -0.17 ± 0.13. This can only be interpreted to mean that in a homogeneous assemblage of families there is no correlation between the grade of booting of parents and offspring.