Mendelian traits are considered as the lower end of the complexity spectrum of heritable phenotypes. However, more than a century after the rediscovery of Mendel’s law, the global landscape of monogenic variants as well as their effects and inheritance patterns within natural populations is still not well understood. Using the yeast Saccharomyces cerevisiae, we performed a species-wide survey of Mendelian traits across a large population of isolates. We generated offspring from 41 unique parental pairs, and analyzed 1,105 cross/trait combinations. We found that 8.9% of the cases were Mendelian. Most were caused by common variants showing stable inheritances in a natural population. However, we also found that a rare monogenic variant related to drug resistance displayed a significant and variable expressivity across different genetic backgrounds, leading to modified inheritances ranging from intermediate to high complexities. Our results illustrate for the first time the continuum of the hidden complexity of a monogenic mutation, where genotype is hardly predictive of phenotype.