Intrafamilial correlation analysis for IgM serum levels.

The IgM serum level was determined in the members of 29 healthy families. The IgM mean concentrations between fathers and mothers and between sons and daughters were significantly different (P less than .01), with higher serum IgM levels in females than in males. Simple linear regression analysis was done for the following intrafamilial combinations: son-father, daughter-father, son-mother, and daughter-mother. Significant correlation coefficients (P less than .05) were obtained in all four combinations, which does not support the X-linked gene hypothesis (i.e., that the X chromosome carries quantitative genes for immunoglobulin M). An alternative explanation for the differences between sexes for IgM serum concentration is considered.