Part 5

It has been suggested that the original material out of which the plus series came consisted of modified individuals produced by a cross with the wild race. This was not known positively to be so, because part of the original stock (with which MacCurdy worked) consisted of hooded black and hooded gray rats captured in company with gray self and black Irish rats and albinos. Subsequent experiments showed that ordinary albino rats, if crossed with wild gray ones, will produce in F₂ all these classes of individuals. This indicated pretty clearly that the particular colony which had fallen into our hands had probably arisen by the crossing of an escaped albino rat with wild ones. But it still remained uncertain what sort of hooded pattern the escaped albino had transmitted and whether or not this had been influenced by the wild cross. We therefore determined to ascertain whether out of our minus series crossed with wild a plus series could be derived. To this end certain of the F₂ extracted hooded individuals (entered as grandchildren in Table 42, row 1, and descended from a single hooded individual of grade -1.87, generation 2½) were mated inter se, thus producing an F₃ generation, Table 44, second row. The selected individuals were the aberrant male of grade +3.50 and females of grade +1.50, so that the mean grade of the chosen parents (extracted from the crossed minus series) was +2.50. They had 34 young ranging in grade from 0 to +3.50, mean +2.06, a regression of 0.44 toward 0, repeating the phenomenon regularly found in both selection series.

In this same experiment some F₂ parents of mean grade -0.75 had 19 young (first row of Table 44), whose mean grade was -0.04, a regression of 0.71 toward 0. We should expect the regression of the offspring of such parents to be less than that of the offspring of the -2.50 parents, and so it would be if it were not for one aberrant individual. Larger numbers of offspring would undoubtedly have given the expected result.

From among the F₃ offspring were chosen parents for the next generation (F₄). The chosen parents ranged in mean grade from +2.25 to +3.12 (Table 45), average +2.52. They produced 205 young ranging in grade from -0.25 to +3.50, mean +1.86, a regression of 0.66.

The parents for the next generation (Table 46) ranged in mean grade from +2.00 to +3.00, the mean being +2.27. They produced 119 offspring of mean grade +2.06, a regression of only 0.21.

TABLE B.—Comparison of the present series with the more general plus selection series.

+----------+------------------------------------------+ | | Present series. | | +--------+----------+-----------+----------+ |Selection.| Mean, | Mean, |Regression.| No. of | | |parents.|offspring.| |offspring.| +----------+--------+----------+-----------+----------+ | 1 | 2.50 | 2.06 | 0.44 | 34 | | 2 | 2.52 | 1.86 | .66 | 205 | | 3 | 2.27 | 2.06 | .21 | 119 | | 4 | 2.69 | 2.41 | .28 | 194 | | 5 | 2.77 | 2.32 | .45 | 97 | | 6 | 3.08 | 2.67 | .41 | 45 | +----------+--------+----------+-----------+----------+

+----------+------------------------------------------+ | | General (plus) series. | | +--------+----------+-----------+----------+ |Selection.| Mean, | Mean, |Regression.| No. of | | |parents.|offspring.| |offspring.| +----------+--------+----------+-----------+----------+ | 1 | 2.51 | 2.05 | 0.46 | 150 | | 2 | 2.52 | 1.92 | .60 | 471 | | 3 | 2.73 | 2.51 | .22 | 341 | | 4 | 3.09 | 2.73 | .36 | 444 | | 5 | 3.33 | 2.90 | .43 | 610 | | 6 | 3.52 | 3.11 | .41 | 861 | +----------+--------+----------+-----------+----------+

The parents chosen from among these offspring ranged in mean grade from +2.37 to +3.25, average +2.69. They produced 194 offspring of grade +0.50 or higher (F₆, Table 47), the range for the first time lying wholly in the plus direction. The mean grade of the offspring was +2.41, a regression of 0.28.

The parents of the next generation (F₇, Table 48) range in mean grade from +2.62 to +3.37, their average being +2.80. Their 154 offspring range from +0.75 to +3.75, mean +2.46, a regression of 0.34.

The parents of the last generation in this experiment (F₈, Table 49) were of mean grade +3.08. They produced 45 offspring of mean grade +2.67, a regression of 0.41.

As a result of a single cross with a wild race followed by six successive selections, a narrow-striped or minus family has thus been converted into a wide-striped or plus family. Considering the smaller number of offspring from which selections could be made, progress was quite as rapid in this series as in the larger plus selection series. The regression is surprisingly similar, generation by generation, in the two series. (See Table B.) But it seems improbable that the closeness of the agreement has any significance. This series has the theoretical advantage of being derived from a single individual of the minus selection series.

CROSSES OF THE PLUS RACE WITH THE MINUS RACE.

When animals of the plus selection series are crossed with animals of the minus selection series, an F₁ generation of offspring is obtained which varies about a mean intermediate between those of the respective uncrossed races. Thus, from an examination of Table 50 it will be seen that when -2 animals of generation 6, minus series, were crossed with +3.50 or +3.75 animals of generation 5, plus series, an F₁ generation (Series 1) was obtained consisting of 93 animals of mean grade +0.06. This generation is rather more variable than either uncrossed race, its standard deviation being 0.71. The same is true of a second set (Series 2) of crosses made between a male of grade -3.25, generation 10, minus series, and females of grade +3.75, generation 10, plus series. The 14 F₁ offspring are of mean grade +1 and have a standard deviation of 0.60. (See Table 50, Series 2.)

In both the series of crosses summarized in Table 50, the F₂ generation is more variable than F₁. In Series 1, 305 F₂ animals are recorded, having a standard deviation of 1.01 as compared with 0.71, the standard deviation of the F₁ generation. In Series 2, the F₂ offspring number 73 and have a standard deviation of 0.87, that of the previous generation being 0.60.

The mean of the F₂ generation is very similar to that of the F₁ generation. In Series 1, the mean of F₁ is 0.06, and that of F₂ is 0.24; in Series 2, the mean of F₁ is 1.00, and that of F₂ is 0.72.

It may also be seen from an examination of Table 50 that among the F₁ offspring produced by crossing the plus and the minus series there are differences in transmission, as there are in the expression of the hooded pattern. In general those F₁ individuals which are of high grade produce offspring of higher grade than do their low-grade brothers and sisters. This is exactly what has been observed in both uncrossed races.

SUMMARY OF RESULTS.

The experiments which have been described in the foregoing pages have shown that:

1. The hooded pattern of rats behaves as a simple Mendelian character in crosses with either the Irish pattern or the wholly pigmented condition of wild rats.

2. Though behaving as a unit, the hooded pattern fluctuates—that is, it is subject to plus and minus variations.

3. Selection, plus or minus, changes the position of the mean and mode about which variation occurs.

4. The results of such plus or minus selections are permanent, for return selection is not more effective than the original selection, and during return selection regression occurs away from the original mode, that is, toward the mode established by selection.

5. During the progress of the original selection (thirteen successive generations) variability as measured by the standard deviation was somewhat diminished.

6. Upon crossing the selected plus and minus races with each other, the variability was somewhat increased in F₁ and was further increased in F₂. The extreme conditions (plus or minus) of the grandparents rarely, if ever, recur in this generation. Only one individual among 378 F₂ young has been recorded in a grade as extreme as either grandparent.

7. Hooded animals extracted in F₂ as recessives from a cross with either Irish or wild rats are as a rule more variable than the selected race used in making the cross. In crosses with an Irish race the minus series was affected in like measure. In crosses with wild rats the variability of the plus series was not appreciably affected (in two experiments it was slightly reduced, and in one experiment it was slightly increased). But the variability of the minus race was more than doubled by crosses with wild rats.

8. The mean of the minus race was lowered by a cross with either the Irish race or with wild rats, but more extensively by the latter. The mean of the plus race was lowered a very little by a cross with wild rats, but considerably by a cross with the Irish race.

DISCUSSION.

It would be possible to suppose, as the senior author has elsewhere suggested (Castle, 1912), that the Mendelian unit character involved in these experiments is subject to quantitative variation and that such quantitative variations have a tendency to persist from generation to generation. This would account for the effectiveness and permanency of selection when brought to bear upon the variations. It might also form a basis for explaining the increased variability which follows crossing, this being regarded as due to contamination in the heterozygote, but there are certain other observed effects of crossing which it seems impossible to account for on this basis. In particular it is observed that while crossing the minus series makes it less minus as the hypothesis of contamination would demand, crossing the plus series makes it less plus, the opposite of what a contamination theory would demand. For we can readily understand, on the basis of contamination, how a +6 gamete being combined with a -2 gamete might change the latter in a plus direction; but if the same +6 gamete is associated with a +4 gamete we should expect it, if it has any influence at all, to make this also more plus, but the observed effect is the opposite; the extracted gametes are less plus in character.