106 Chapter 3 • The Chromosomal Basis of Inheritance shown in Figure 3-45. Note that the condition is always passed on through mothers and never fathers. Occasion-ally a mother will produce an unaffected child (not shown), probably re±ecting cytoplasmic segregation in the gamete-forming tissue. Cytoplasmic segregation of accumulated deleterious mutations in certain tissues such as muscle and brain has been proposed as a mecha-nism of aging. In certain special systems, cytohets that are “di-hybrid” have been obtained (say, AB in one organelle chromosome and ab in another). In such cases, rare crossover-like processes can occur, but this must be con-Figure 3-45 Pedigree showing maternal inheritance of a human mitochondrial disease. I II III MESSAGE Alleles on organelle chromosomes 1. In sexual crosses are inherited from one parent only (generally the maternal parent) and hence show no segregation ratios of the type nuclear genes do 2. Asexual cells can show cytoplasmic segregation
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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.