What are three exceptions of Mendel's observations?

Three exceptions to Mendel's observations include incomplete dominance, where heterozygous individuals express a phenotype that is a blend of the two alleles; codominance, where both alleles in a heterozygote are fully expressed, such as in AB blood type; and polygenic inheritance, where multiple genes influence a single trait, resulting in a continuous range of phenotypes, as seen in human skin color. These exceptions highlight the complexity of genetic inheritance beyond Mendel's original principles.