A hypostatic gene is one whose phenotype is altered by the expression of an allele at a separate locus, in an epistasis event.
Example: In labrador retrievers, the chocolate coat colour is a result of homozygosity for a gene that is epistatic to the "black vs. brown" gene. The alleles determining whether the dog is black or brown, are that of the hypostatic gene in this event.
Book: Hartwell , Leland . L. Hood . M. Goldberg . A. Reynolds . L. Silver . R. Veres . Genetics: From Genes to Genomes . McGraw-Hill . 2004 . New York, NY . 56–57 . 0-07-246248-5 .