Wikipedia
Midparent
Studying quantitative traits in heritability studies may be complicated by sex differences observed for the trait. The midparent value is defined as the average of the trait value of father and mother. This value can be used in the study to analyze the data set without heeding sex effects.
Well-known examples include studies of stature height which is given by:
$height_{midparent}=\frac{1}{2}(height_{father}+(1.08\times height_{mother}))$[1]
The coefficient 1.08 serves as a scaling factor. After the 1.08 scaling, the mean of the mother's height is the same as that of the father's, and the variance is closer to the father's. In this way, sex difference can be ignored.