Since a mutation such as poky must have arisen originally in one mitochondrial “chromosome,” how can it come to occupy all the mitochondrial chromosomes in a poky mutant? The process, which is quite common among organelle mutations, is not well understood. In some cases it appears to be a series of random chances. In other cases, the mutant chromosome seems to possess some competitive advantage in replication. In some cases, cells contain mixtures of mutant and normal organelles. These cells are called cytohets or het-eroplasmons . In these mixtures, a type of cytoplasmic segregation can be detected, in which the two types apportion themselves into different daughter cells. The process most likely stems from chance partitioning dur-ing cell division. Plants provide a good example. Many cases of white leaves are caused by mutations in chloroplast genes that control the production and dep-osition of the green pigment chlorophyll. Since chloro-phyll is necessary for the plant to live, this type of mu-tation is lethal, and white-leaved plants cannot be obtained for experimental crosses. However, some
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This note was uploaded on 01/10/2011 for the course BIOL BIOL taught by Professor Johnson during the Spring '08 term at Aberystwyth University.