Mendel's laws laid the foundation for classical genetics by describing inheritance of traits controlled by nuclear genes. However, many traits do not follow Mendelian ratios. These deviations are collectively called non-Mendelian inheritance. A major class of non-Mendelian inheritance is cytoplasmic (extranuclear) inheritance, where genes in cytoplasmic organelles such as mitochondria and chloroplasts determine phenotypes.
Heterozygotes show an intermediate phenotype (example: pink flowers of Mirabilis jalapa when red is crossed with white).
Both alleles express equally in heterozygotes (example: ABO blood groups in humans — type AB expresses both A and B antigens).
More than two allelic forms exist for a locus (example: ABO blood group has three alleles: IA, IB, i).
Certain allele combinations are lethal and alter expected Mendelian ratios (example: yellow coat in mice when homozygous may be lethal).
Genes on the same chromosome may be inherited together; recombination frequency reflects crossover events.
One gene may mask or modify the expression of another (examples: complementary, duplicate, recessive epistasis).
Traits governed by organelle genomes (mitochondrial and chloroplast DNA) that usually show maternal transmission and non-Mendelian ratios.
Cytoplasmic inheritance is heredity in which genes located in the cytoplasm — principally in mitochondria and chloroplasts — determine phenotypes. Because the egg contributes most of the cytoplasm to the zygote, such traits are commonly maternally inherited.
- Maternal inheritance: Offspring phenotype depends on the mother's cytoplasm (egg) rather than the pollen or sperm in most organisms.
- Non-Mendelian ratios: Segregation ratios like 3:1 or 9:3:3:1 are not observed.
- Organelle genomes: Involves mtDNA and cpDNA which are usually circular and replicate independently.
- Variegation/mosaicism: Many cytoplasmic mutations cause mosaic phenotypes (patches of different colors).
- Random segregation of organelles: Organelles distribute stochastically to daughter cells, leading to phenotypic heterogeneity.
- Uniparental or biparental transmission: Mostly maternal, but exceptions exist where paternal or biparental inheritance occurs.
Mitochondria have their own DNA encoding components of the respiratory chain. Mutations lead to defects in energy metabolism and cause mitochondrial diseases.
Chloroplasts contain DNA required for photosynthesis and chloroplast biogenesis. Mutations in plastid DNA cause defects in chlorophyll synthesis and variegation.
Variegation (green, white or mottled leaves) in Mirabilis is a classic example. The phenotype of a seedling depends on the cytoplasm of the maternal parent (the ovule) and not on the pollen donor. This arises from plastid mutations affecting chlorophyll production.
Yeast cells with defective mitochondrial DNA form small colonies called "petite". Their inheritance is non-Mendelian because of mitochondrial involvement and maternal cytoplasmic transmission.
CMS, controlled by mitochondrial genes, prevents pollen formation in many crops (maize, sunflower, rice). CMS is valuable in hybrid seed production because it eliminates the need for manual emasculation.
Mitochondrial disorders (e.g., LHON — Leber's hereditary optic neuropathy, MELAS) typically show maternal inheritance because mitochondria are transmitted through the egg. Heteroplasmy (mixture of normal and mutant mtDNA) affects disease severity and penetrance.
| Feature | Nuclear (Mendelian) | Cytoplasmic (Non-Mendelian) |
| Location of genes | Nucleus (chromosomes) | Cytoplasmic organelles (mitochondria, chloroplasts) |
| Inheritance pattern | Mendelian ratios (segregation & independent assortment) | Usually maternal; non-Mendelian ratios |
| Parental contribution | Both parents contribute equally (nuclear genes) | Mostly maternal (egg provides cytoplasm) |
| Segregation | Predictable by Mendel's laws | Random distribution of organelles; heteroplasmy possible |
- Explains many deviations from Mendelian expectations observed in organisms.
- Used in agriculture: CMS is exploited to produce hybrids efficiently in many crop species.
- Critical for human medicine: understanding mtDNA diseases and inheritance patterns helps in diagnosis and counseling.
- Useful in evolutionary studies: mitochondrial DNA is widely used for tracing maternal lineages and phylogenetics.
