Part 7

+-------------------------------------+-------------------+------+------+ | |Standard deviation | | | | | of races crossed, | S. D.| S. D.| | | same generation. | F₁ | F₂ | +-------------------------------------+---------+---------+------+------+ |Plus-minus cross, series 1 (Table 50)| 0.49 | 0.50 | 0.71 | 1.01 | |Plus-minus cross, series 2 (Table 50)| .36 | .24 | .60 | .87 | |Mutant-minus cross, lower group | .25 | .24 | .77 | 1.17 | |Mutant-minus cross, upper group | .19 | .24 | .31 | .44 | |Mutant-plus cross, lower group | .25 | .36 | .24 | .35 | |Mutant-plus cross, upper group | .19 | .36 | .23 | .15 | +-------------------------------------+---------+---------+------+------+

The mutant-plus cross, it will be observed, shows no increase of variability either in F₁ or in F₂, but crosses involving the minus race show increase of variability both in F₁ and in F₂. Interpreted on a Mendelian basis, this means that the mutant and plus races on the one hand and the minus race on the other hand differ by more than a single factor. If they differed by only a single factor, then crosses between them should bring no increase of variability, either in F₁ or in F₂. This appears to be true as regards the mutant and plus races when crossed with each other. But if the races crossed differ by more than one factor, and if, further, neither parent is homozygous as regards the factors in which they differ, then we may expect an increase in variability both in F₁ and in F₂. This is exactly what we observe when the minus race is crossed with either the plus race or its derivative, the mutant race.

If we suppose that the plus race and the minus race differ from each other by certain “modifiers,” we can not suppose that the plus and the mutant races differ by these same modifiers. They differ in some other single respect; perhaps that in which they differ is the main hooded factor. Are we, then, to suppose that the plus and the minus races do not differ as regards this same main factor? This can not be stated, but we see no reason for considering them identical as regards that factor. It appears that the mutant race arose from the plus race by a single large plus variation, which seems to have its determiner in some single component of the germ-cell. But the fact that this change came as a large quantitative variation does not show that small variations are impossible in that same cell component. It seems to us quite improbable that the plus mutation could have arisen in the minus selection series. We believe that the repeated selection which was practised had something to do with inducing this change in the plus direction. If one can increase at will the “modifiers” which make the pigmentation more extensive, it does not seem strange that after a time a readjustment should occur within the cell which should incorporate modifiers in that part of the cell which is responsible for the unit-character behavior of the hooded pattern. This would amount to a quantitative change in the unit-character for hooded pigmentation.

BIBLIOGRAPHY.

CASTLE, W. E.

1905. Heredity of coat characters in guinea-pigs and rabbits. Carn. Inst. Wash. Pub. 23.

1906. The origin of a polydactylous race of guinea-pigs. Carn. Inst. Wash. Pub. 49.

1912. The inconstancy of unit-characters. American Naturalist, vol. 46, pp. 352-362.

CASTLE, W. E., and ALEXANDER FORBES.

1906. Heredity of hair-length in guinea-pigs and its bearing on the theory of pure gametes. Carn. Inst. Wash. Pub. 49.

DE VRIES, H.

1901-1903. Die Mutationstheorie. Veit & Co., Leipzig.

EAST, E. M.

1912. The mendelian notation as a description of physiological facts. American Naturalist, vol. 46, pp. 633-655.

JENNINGS, H. S.

1909. Heredity and variation in the simplest organisms. American Naturalist, vol. 43, pp. 321-337.

1910. Experimental evidence on the effectiveness of selection. American Naturalist, vol. 44, pp. 136-145.

JOHANNSEN, W.

1909. Elemente der exakten Erblichkeitslehre. G. Fischer, Jena.

MACCURDY, H., and W. E. CASTLE.

1907. Selection and cross-breeding in relation to the inheritance of coat-pigments and coat-patterns in rats and guinea-pigs. Carn. Inst. Wash. Pub. 70.

PEARL, R.

1913. Genetics and breeding. Science, n. s., vol. 37, pp. 539-546.

TABLES.

TABLE 1.—Classification of the first generation of offspring in the plus selection series. At the head of each column is indicated the grade of the individuals recorded in that column. The figures in the body of the table indicate the numbers of offspring of the several grades indicated.

+--------+--------------------------+-------+------+-----------+ | Grade | Grade of offspring. | | | | | of +--+--+--+--+--+--+--+--+--+Totals.|Means.|Regression.| |parents.|+1|1¼|1½|1¾| 2|2¼|2½|2¾| 3| | | | +--------+--+--+--+--+--+--+--+--+--+-------+------+-----------+ | 1⅞ | 1| 1| 1| 1| 1| | 1| 1| | 7 | 1.82 | .05 | | 2 | 4| 1| 3| 1| 4| 1| 4| | | 18 | 1.76 | .24 | | 2⅛ | | | | | | | | | | | | | | 2¼ | 3| 1| 3| 1| 6| 2| 2| 2| | 20 | 1.87 | .38 | | 2⅜ | | | | | | | | | | | | | | 2½ | 5| | 3| 1|13| 1| 8| 5| 1| 37 | 2.06 | .44 | | 2⅝ | | | | | | 1| 1| 2| 1| 5 | 2.15 | .47 | | 2¾ | 7| | 3| 2|17| 1|12| | 9| 51 | 2.12 | .63 | | 2⅞ | | | | | | | | | | | | | | 3 | | | | 2| 3| 1| 3| | 3| 12 | 2.35 | .65 | | +--+--+--+--+--+--+--+--+--+-------+------+-----------+ |Totals | | | | | | | | | | | | | | or | | | | | | | | | | | | | |means, | | | | | | | | | | | | | | 2.51|20| 3|13| 8|44| 7|31|10| 4| 150 | 2.05 | .46 | +--------+--+--+--+--+--+--+--+--+--+-------+------+-----------+

TABLE 2.—Classification of offspring in generation 2, plus selection series.

+--------+-----------------------------------------------------------+ | Grade | Grade of offspring. | | of +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |parents.|-1|-¾|-½|-¼| 0|+¼| ½| ¾| 1|1¼|1½|1¾| 2|2¼|2½|2¾| 3|3¼|3½|3¾| +--------+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 2 | | | 1| | 1| | | 4| 2| 5| 7| 3| 7| 8| 2| 3| 2| 1| | | | 2⅛ | 1| | 1| | 2| 1| 3| 5|12| 8| 5| 5| 8| 2| 1| 2| | | | | | 2¼ | | | | | | | | | | 1| 1| 3| 1| 1| | 1| | | | | | 2⅜ | | | | | | | | | 1| 1| 4| 9| 7| 8| 8| 6| 1| | | | | 2½ | | | | | | | | 4|22|16|19|11|21|13| 7|10| 9| 1| | | | 2⅝ | | | | | 1| | 2| | 3| 1| | 6| 5| 7| 9| 6| 3| 1| | | | 2¾ | | | | 1| | | | 3| 4| 2| 6| 5| 9| 6| 9| 6| 1| | | | | 2⅞ | | | | | | | | | | 1| | 1| 3| 5| 3| 9| 1| | | | | 3 | | | | | | | | | 1| 2| 3| 2|14| 7| 4| 5|11| 6| 3| 1| | 3½ | | | | | | | | | | | | | 2| | 1| | 2| | | | | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |Totals | | | | | | | | | | | | | | | | | | | | | | or | | | | | | | | | | | | | | | | | | | | | |means, | | | | | | | | | | | | | | | | | | | | | | 2.52| 1| -| 2| 1| 4| 1| 5|16|45|37|45|45|77|57|44|48|30| 9| 3| 1| +--------+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+