INBRED AND HYBRID ANIMALS AND THEIR VALUE 333 



Measurement of Phenotypic Variation 



In order to discuss the phenotypic effects of inbreeding it is necessary 

 to express phenotypic variation in terms that the mind can grasp. The two 

 most useful measures of a distribution of biological data are its location 

 and scatter. Thus, the two most significant questions that can be asked 

 about a set of measurements on, for example, tail length in a population of 

 mice are: (7) Where is the distribution of lengths located? or: What is the 

 average length? and (2) How much spread is there about this average? It 

 will be convenient to refer to these two measures in a general sense as 

 "average" and "variation," remembering that each can be expressed in 

 definite terms by several statistics, of which the mean and standard devia- 

 tion are respectively the most valuable. 



We can now separate the phenotypic effects of inbreeding into effects on 

 the average and eft'ects on the variation. The discussion of some of these 

 will be simplified if we assume that the genetic effects have reached the 

 limiting condition in which there is no genetic variation left, all the indi- 

 viduals in the inbred line having one and the same genotype. Such a limit 

 has actually been reached, at least for all genes with measurable effects on 

 certain characters in certain inbred lines. If, through mutation or insufii- 

 cient inbreeding, a line is not genetically pure for a certain character, the 

 effects discussed below may still occur, although perhaps not to the full 

 extent possible. 



Effect of Inbreeding on the "Average" 



The average value of a character will be prescribed by the genotype 

 fixed. It seems extremely likely that this genotype will determine an 

 average that differs from that in the foundation stock from which the 

 inbred line was derived. There will be no change if: (i) the character 

 is not affected by the extent of genetic variation possible with the original 

 genes, or (2) the effect of the genotype fixed happens to correspond to the 

 average effect of the many genotypes present in the foundation stock. 

 Neither condition can be expected to occur very often. Observation agrees 

 with expectation: a change in average usually occurs with inbreeding and is, 

 as we shall see later, one of the most valuable results for the research worker. 



With many characters the change in average may go in either direction. 

 For example, average amount of white spotting in a piebald stock may either 

 increase or decrease with inbreeding. However, with some characters the 

 change is in the same direction in all, or most, of several inbred lines studied. 



