Scientists discover inherited traits that break Mendel’s Laws of genetics

Researchers at Johns Hopkins Medicine and Texas A&M University have identified hundreds of epigenetic inheritance patterns in mice that defy Gregor Mendel’s classical genetic laws. Published May 20 in *Nature Genetics*, the study found that about 7% of these patterns behaved unexpectedly, including cases where epigenetic marks appeared without clear parental origins. The team also documented the first naturally occurring paramutation in a mammal, a phenomenon previously observed only in plants and flies. The findings challenge the long-held assumption that genetic inheritance follows predictable Mendelian rules. While Mendel’s laws describe how alleles (gene variants) are passed from parents to offspring, epigenetic modifications—such as DNA methylation—can alter gene activity without changing the underlying DNA sequence. These chemical marks, often influenced by environmental factors, were previously known to occasionally override Mendelian inheritance, but the new study suggests such deviations may be more common than recognized. The researchers examined DNA methylation in tissue samples from three generations of mice, aged 4–6 months. They discovered five additional genes subject to genomic imprinting, where methylation silences an allele depending on its parental origin rather than dominance. Additionally, some epigenetic patterns could not be traced to either parent, hinting at unknown mechanisms of inheritance. Lead author Andrew Feinberg, M.D., noted that non-Mendelian epigenetic inheritance could allow organisms to rapidly acquire new traits in response to environmental pressures. The study was supported by the National Institutes of Health and the National Science Foundation, underscoring its potential implications for understanding heredity beyond genetic sequences.