Chapter 5: Non-Mendelian Inheritance

Maternal effect genes represent a foundational concept where the mother's genotype—rather than the offspring's own genetic makeup—determines the offspring's phenotype during early development. This occurs because the maternal nucleus contributes messenger RNAs and proteins into the egg cytoplasm that regulate embryonic development before the offspring's own genome becomes transcriptionally active. The inheritance of shell coiling direction in snails exemplifies this principle, as the mother's genotype controls the offspring's coiling pattern regardless of the offspring's genotype. The chapter then explores epigenetic inheritance mechanisms, particularly dosage compensation and genomic imprinting, which modify gene expression without altering DNA sequences. Dosage compensation equalizes gene expression between males and females despite their different sex chromosome compositions; in mammals, this involves X-chromosome inactivation where one X chromosome in female cells condenses into a transcriptionally inactive Barr body, creating the mosaic phenotype seen in tortoiseshell cats. Genomic imprinting involves parent-of-origin-specific gene silencing through DNA methylation and chromatin remodeling, ensuring certain alleles are expressed only when inherited from a particular parent, as demonstrated by the insulin-like growth factor two gene in mice. The chapter further addresses mitochondrial inheritance, where traits encoded by mitochondrial DNA pass exclusively through maternal lineages because sperm contribute negligible cytoplasm to the zygote. Heteroplasmy—the coexistence of normal and mutated mitochondrial DNA copies within individual cells—explains variable disease severity in mitochondrial disorders. Together, these mechanisms illustrate that inheritance patterns depend not only on chromosomal genetics but also on cytoplasmic contributions, epigenetic modifications, and parent-of-origin effects, fundamentally expanding our understanding of how traits are transmitted across generations.