NonMendelian

Non-Mendelian inheritance refers to patterns of heredity that do not follow Mendel's laws of segregation and independent assortment. These patterns yield offspring phenotypes that deviate from the classic 3:1 or 9:3:3:1 ratios and arise from diverse genetic and epigenetic mechanisms.

Key mechanisms include cytoplasmic inheritance, in which organellar genomes such as those of mitochondria or chloroplasts are transmitted primarily via the egg, producing maternal inheritance patterns and potential heteroplasmy. Genomic imprinting involves parent-of-origin–specific expression due to epigenetic marks established in gametes. Incomplete dominance and codominance yield heterozygote phenotypes that are intermediate or distinctly mixed rather than strictly dominant or recessive. Polygenic and quantitative traits show continuous variation. Epistasis, where one gene modifies another's effect, and pleiotropy, where a single gene influences multiple traits, further complicate inheritance. Linkage and reduced independent assortment can bias expected ratios when genes are physically linked.

In practice, distinguishing non-Mendelian effects from sampling error, small pedigrees, or testing limitations can be difficult. Apparent deviations may reflect statistical variation or population structure rather than a true non-Mendelian mechanism. Understanding these patterns has implications for genetic counseling, breeding, and studies of evolution and development.

See also: cytoplasmic inheritance, genomic imprinting, epigenetics, polygenic inheritance, epistasis, pleiotropy, incomplete dominance, codominance.