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Piebald rats and selection

Chapter 6: MINUS SELECTION SERIES.
By William E. Castle
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About This Book

Investigators present a controlled breeding study that tests whether Mendelian determiners remain unchanged during crosses by selecting for increased or decreased pigmentation in piebald coat patterns of laboratory rats. They describe methods for grading dorsal pigmentation, establish parallel plus and minus selection lines, perform return and hybrid crosses with different stocks, and tabulate multigenerational outcomes. Statistical summaries and plates illustrate shifts in pigment distribution and the response to selection. The results are analyzed with respect to the theory of pure gametes and genotype–phenotype distinctions, showing that extracted recessive patterns and selection responses complicate simple notions of gametic purity.

MINUS SELECTION SERIES.

This series begins with selected parents ranging in grade from -1.25 to -1.87. Their average, if each pair is weighted in proportion to the number of its offspring, is -1.46. The offspring (Table 16), like the offspring of the original plus selections, regress toward grade 0. They range in grade from +0.25 to -2.00, their mean being -1.00. The total number of offspring recorded in this generation is only 55, this being too small to warrant the calculation of a correlation coefficient.

Generation 2 (Table 17) is somewhat larger, but still too small to make statistical constants based upon it of much consequence. The offspring show substantially the same range of variation as in the previous generation, but with a slightly higher average (-1.07). The coefficient of correlation (-0.03) is negative, but too small to be significant. The record of the next eleven generations will be found summarized in Tables 18 to 28, or in more condensed form in Tables 29 and 30. Generation 13 (Table 28) is still incomplete.

The mean of the parents steadily rises from -1.56 in generation 3 to -2.50 in generation 13. The mean of the offspring rises by like increments from -1.18 in generation 3 to -2.39 in generation 13. There is throughout these generations a positive correlation between parents and offspring. This amounts on the average to 0.137 as compared with 0.193 observed in the plus selection series. The absolute change in amount of pigmentation is no doubt less in the minus selection than in the plus selection series, but if the change were recorded as percentage decrease of pigmentation in one case and percentage increase in the other, the change indicated would probably be as great in one as in the other.

In the minus as in the plus series we observe:

(1) The character of the offspring varies with that of the parents; high-grade parents have high-grade offspring and vice versa.

(2) The variability of the race (as indicated by the standard deviation) undergoes some reduction and the limits of variation, both upper and lower, are displaced in the direction of the selection.

(3) The regression from a new and extreme class of parents is at first large, but decreases as the selection is repeated and finally disappears altogether when the average of the race becomes equal to the particular grade under discussion.