Extra Chromosomal / Cytoplasmic Inheritance

Not every inherited character is controlled by a gene sitting on a nuclear chromosome. Certain characters instead depend on genomes found outside the nucleus altogether — in the chloroplast, in the mitochondrion, in infective cytoplasmic agents, or in plasmids — and inheritance of these characters does not follow the familiar Mendelian segregation ratios at all.

The reason this class of inheritance shows a maternal bias is straightforward: although a sperm and an egg contribute equally to the zygote's nuclear (chromosomal) genes, they do not contribute equally to its cytoplasm. A mature sperm carries almost no cytoplasm of its own, while the egg supplies essentially the entire initial cytoplasm and organelle complement of the new zygote. Any hereditary units riding in that cytoplasm — rather than in the nucleus — are therefore transmitted overwhelmingly through the mother, a pattern called maternal effect. Because male and female parents end up contributing unequally to these extra-chromosomal genes (even while contributing equally to the nuclear genome), crosses involving cytoplasmically-inherited traits can produce results that look distinctly non-Mendelian — reciprocal crosses (swapping which parent is male and which is female) can give different results, which is never true of ordinary nucle …