Knowledge of the time and manner of origin of genetically induced defects may well be sought by classical methods of experimental embryology, involving transplantations between genotypes, providing the effects on a tissue of residence in a host of different genotype is not obscured by generalized reactions to transplantation. Such experimental transplantation is reported here, applied to analysis of the pleiotropic effects of the deleterious alleles at the W-locus in the mouse (Russell, review, 1955).

Through extensive investigation it has become apparent that the three major types of defect associated with these alleles are already established at birth. In contrast to their normal (ww) littermates, new-borns of the genotypes with which this paper will largely be concerned (WVWV and WW) are severely anaemic (Russell & Fondal, 1951), their gonads are almost totally devoid of germ-cells (Coulombre & Russell, 1954), and their hair follicles lack melanoblasts (Silvers, 1953).

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