In such circumstances, the data must be reconstructed so as to eliminate the effect of one character, g , upon the other, h . Unless the two characters, g and h , are of the same frequency, the result of adjusting h for the effect of g will differ from the result obtained when g is adjusted to eliminate the effect of h . The practical question as to which adjustment to make must be decided after considering the nature of the material. The same type of analysis can be applied to characters which segregate but whose genetic behaviour is not known precisely. Now follows a simple worked example.
Eye colour is a graded character but it can roughly be measured by three categories.
By examination of a series of pairs of sibs in this instance from different families we propose to test whether there is linkage between eye colour and sex, i.e. whether a gene which causes significant variation in eye colour is situated on a sex chromosome. Without making any adjustments, The observed value of f is of a magnitude which suggests the likelihood of linkage but it is not significantly greater than its standard error. The adjustment does not alter the result appreciably. If the genetic behaviour, of a graded character which is used in the linkage test, is not known, and if a significant value of f is obtained, the inference can be drawn that a gene responsible for a major variation in this character is located close to the gene responsible for variation in that character with which it is compared. Graded human characters are subject to variability due to effects of subsidiary genes or to environmental influences.