什么是 Extranuclear inheritance？

Mitochondria and chloroplasts carry their own small genomes, in many copies per organelle and many organelles per cell, and they come almost entirely from the egg: the sperm contributes a nucleus and little else. Their genes are therefore maternally inherited, without segregation ratios: a mother passes the trait to all her children, a father to none. Human mitochondrial diseases (defects of the respiratory chain affecting muscle and nerve) follow this pattern, and so does the variegation of some plants, where a mother cell containing both normal and mutant chloroplasts sorts them at random into daughter cells, producing green, white and mixed sectors. Because they never recombine, mitochondrial sequences trace maternal lineages back through time — the tool of Chapter 24.