Methods for using genetic marker genotype (e.g., gene sequence diversity information) to improve the process of developing plant varieties (e.g., single cross hybrids) with improved phenotypic performance are provided. Methods for predicting the value of a phenotypic trait in a plant are provided. The methods use genotypic, phenotypic, and optionally family relationship information for a first plant population to identify an association between at least one genetic marker and the phenotypic trait, and then use the association to predict the value of the phenotypic trait in one or more members of a second, target population of known marker genotype. Methods for identifying new allelic variants affecting the trait are also provided. Plants selected, provided, or produced by any of the methods herein, transgenic plants created by any of the methods herein, and digital systems for performing the methods herein are also provided.