QUANTITATIVE INHERITANCE 



197 



The relationship in equation (4) shows that the precision of an estimate of the 

 degree of genetic determination depends on the value ofg itself. The higher the value, 

 the greater the precision with a given number of animals measured. To plan an 

 experiment it is therefore necessary to guess what the degree of genetic determination 

 may be. This is not very satisfactory, but it is better than working completely in the 



Fig. 34. Graphs of y — \/2/x and y = 2/Vx. 



■10 



(a) Graph of y — V2/x; (b) y = 2/Vx. Two scales are given: the left hand scale of 

 y is to be read against the bottom scale of x, and the right hand against the top. 



Uses 



1. Standard error of estimate of a variance, V, (graph a) : a v — yV, when x is the 

 number of degrees of freedom. 



2. Standard error of estimate of the degree of genetic determination, g, by comparison 

 of variances of genetically uniform and genetically heterogeneous groups with equal numbers 

 in each group (graph b) : a g = y{\ — g), when x is the number of degrees of freedom in 

 each group. 



3. Standard error of estimate of heritability by regression of offspring on parents, with 

 one offspring from each parent or pair of parents, y is the standard error when x is the 

 number of parents or pairs of parents. Graph a refers to regression on the mean of both 

 parents, graph b to the regression on one parent. The inset graphs show the effect of in- 

 creasing the number of offspring when the number of parents remains the same. The 

 vertical scale gives the factor by which y (from graphs a and b) is to be multiplied to give the 

 standard error. The four inset graphs refer to different characters with phenotypic correla- 

 tions (t) between offspring of the same parents as shown. 



