105 3.5 Organelle chromosomes generations of cell division to be complete (Figure 3-43). Hence as a result of these chance events both alleles are expressed in different daughter cells, and this separation will continue through the descendants of these cells. This type of genetic segregation is called cytoplasmic segregation. Note that it is not a mitotic process; it takes place in dividing asexual cells, but it is unrelated to mito-sis. In chloroplasts, cytoplasmic segregation is a common mechanism for producing variegated (green/white) plants, as we saw in Chapter 2. In the poky mutant of the fungus Neurospora, also introduced in Chapter 2, the original mutation in one mtDNA molecule must have accumulated and undergone cytoplasmic segregation to produce the strain expressing the poky symptoms. Figure 3-44 shows some of the mutations in human mitochondrial genes that can lead to disease when, by random drift and cytoplasmic segregation, they rise in frequency to such an extent that cell function is impaired. The inheritance of a human mitochondrial disease is
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