Part 8 (2/2)
42
32
38
34
16
16
8
4
3
Length of{ F_1
1
2
1
1
1
2
2
5
3
ulna { B.C.
12
11
20
26
17
19
18
15
12
13
15
11
5
_same table continued_
CHARACTER
GENERATION
13
14
15
16
17
18
19
20
21
22
23
24
25
---------+----------+---+---+---+---+---+---+---+---+---+---+---+---+---+ Length of{ F_1
3
2
2
skull { B.C.
1
Length of{ F_1
1
7
3
2
1
2
1
1
ulna { B.C.
2
4
2
2
1
1
He found that the variability was smaller in the first generation than in the second generation (back cross). This is what is expected if several factor-differences were involved, because the hybrids of the first generation are expected to be more uniform in factorial composition than are those in the second generation which are produced by recombination of the factors introduced through their grandparents. Excellent ill.u.s.trations of the same kinds of results have been found in Indian corn. As shown in figure 85 the length of the cob in F_1 is intermediate between the parent types while in F_2 the range is wider and both of the original types are recovered. East states that similar relations have been found for 18 characters in corn. Emerson has recently furnished further ill.u.s.trations of the same relations in the length of stalks in beans.
[Ill.u.s.tration: FIG. 85. Cross between two races of Indian corn, one with short cobs and one with long cobs. The range of variability in F_1 is less than that in F_2. (After East.)]
A similar case is shown by a cross between fantail and common pigeons (fig.
86). The latter have twelve feathers in the tail, while the selected race from which the fantails came had between 28 and 38 feathers in the tail.
The F_1 offspring (forty-one individuals) showed (fig. 87) between 12 and 20 tail feathers, while in F_2 the numbers varied between 12 and 25. Here one of the grand-parental types reappears in large numbers, while the extreme of the other grand-parental type did not reappear (in the counts obtained), although the F_2 number would probably overlap the lower limits of the race of fantail grandparents had not a selected (surviving) lot been taken for the figures given in the table.
[Ill.u.s.tration: FIG. 86. Cross of pigeon with normal tail P_1 and fantail P_1; F_1, bird below.]
[Ill.u.s.tration: FIG. 87. Cross of normal and fantail pigeons. (See Fig. 86.) The F_2 range is wider than that of F_1. The normal grand-parental type of 12 feathers was recovered in F_2 but the higher numbers characteristic of fantails were not recovered.]
The preceding account attempts to point out how I should prefer to interpret the problem of selection in the light of the most recent work on breeding. But I would give a very incomplete account of the whole situation if I neglected to include some important work which has led some of my fellow-workers to a very different conclusion.
[Ill.u.s.tration: FIG. 88. Scheme to show cla.s.ses of hooded rats used by Castle. (After Castle.)]
Castle in particular is the champion of a view based on his results with hooded rats. Starting with individuals which have a narrow black stripe down the back he selected for a narrower stripe in one direction and for a broader stripe in the other. As the diagram shows (fig. 88) Castle has succeeded in producing in one direction a race in which the dorsal stripe has disappeared and in the other direction a race in which the black has extended over the back and sides, leaving only a white mark on the belly.
Neither of these extremes occurs, he believes, in the ordinary hooded race of domesticated rats. In other words no matter how many of them came under observation the extreme types of his experiment would not be found.
Castle claims that the factor for hoodedness must be a single Mendelian unit, because if hooded rats are crossed to wild gray rats with uniform coat and their offspring are inbred there are produced in F_2 three uniform rats to one hooded rat. Castle advances the hypothesis that factors--by which he means Mendelian factors--may themselves vary in much the same way as do the characters that they stand for. He argues, in so many words, that since we judge a factor by the kind of character it produces, when the character varies the factor that stands for it may have changed.
As early as 1903 Cuenot had carried out experiments with spotted mice similar to those of Castle with rats. Cuenot found that spotted crossed to uniform coat color gave in F_2 a ratio of three uniform to one spotted, yet selection of those spotted mice with more white in their coat produced mice in successive generations that had more and more white. Conversely Cuenot showed that selection of those spotted mice that had more color in their coat produced mice with more and more color and less white. Cuenot does not however bring up in this connection the question as to how selection in these spotted mice brings about its results.
Without attempting to discuss these results at the length that they deserve let me briefly state why I think Castle's evidence fails to establish his conclusion.
In the first place one of the premises may be wrong. The three to one ratio in F_2 by no means proves that all conditions of hoodedness are due to one factor. The result shows at most that one factor that gives the hooded types is a simple Mendelian factor. The changes in this type may be caused by modifying factors that can show an effect only when hoodedness is itself present. That this is not an imaginary objection but a real one is shown by an experiment that Castle himself made which furnishes the ground for the second objection.
Second. If the factor has really changed its potency, then if a very dark individual from one end of the series is crossed to a wild rat and the second generation raised we should expect that the hooded F_2 rats would all be dark like their dark grandparent. When Castle made this test he found that there were many grades of hooded rats in the F_2 progeny. They were darker, it is true, as a group than were the original hooded group at the beginning of the selection experiment, but they gave many intermediate grades. Castle attempts to explain this by the a.s.sumption that the factor made pure by selection became contaminated by its normal allelomorph in the F_1 parent, but not only does this a.s.sumption appear to beg the whole question, but it is in flat contradiction with what we have observed in hundreds of Mendelian cases where no evidence for such a contamination exists.
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