 Mendelian methods of inference based on a small set of traits and pairs of genes were complicated by phenomena that came to be called epistasis, expressivity, penetrance, and incomplete dominance and, to a lesser extent, by a background level of mutation for any gene being studied. Muriel Wheldale’s genetic analysis of the color of snapdragon flowers, for example, showed that plants with one or more dominant alleles (i.e., variants of the gene) at a certain locus would show color patterns that she was able to associate with the genotype at three other loci, but plants with two recessive alleles at the first locus would be white no matter what—the homozygote recessive genotype had an epistatic effect over the other genotypes. A range of phenotypes may be shown to correspond to the same genotype—expressivity. A phenotype that is associated with a certain genotype may be observed for only a fraction of individuals in or with that genotype—penetrance. With respect to expressivity and penetrance, researchers try to link the observed variation to conditions occurring during development, stochastic developmental noise, or differences remaining at loci not under study, and to decide where in the range of the trait, say, melanin pigmentation, to demarcate one phenotype from another. Incomplete dominance means the occurrence of an intermediate phenotype (e.g., pink snapdragon flowers resulting from crosses of white and red inbred lines). 