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Modern Genetic Analysis Chapter 4. The Inheritance of Genes
Inheritance of Organelle Genes

Mitochondria and chloroplasts are specialized organelles located in the cytoplasm. They contain a defined subset of the total cell genome (mitochondrial genes are shown in Figure 4-28). These genes show their own special mode of inheritance. In a cross, both parents contribute equally to the nuclear genome of the zygote. However, the cytoplasmic contribution of the male and the female parent is generally unequal; the egg contributes the bulk of the cytoplasm and the sperm essentially none. Because organelles reside in the cytoplasm, the organelle genes generally show strictly maternal inheritance. In other words, essentially none of the organelle DNA in the zygote is from the male parent. A simple example is seen in the inheritance of the Neurospora slow-growing mutant poky, which is caused by a defect in one of the mitochondrial genes. Neurospora can be crossed in such a way that one parent acts as the maternal and the other the paternal parent. In the cross of a poky female with a normal male, the progeny are all poky (Figure 4-29), the precise inheritance pattern expected from a mitochondrial gene.

Figure 4-30 on page 120 demonstrates maternal inheritance in the four-o'clock plant. The color of the chloroplasts in this plant determines the color of the various branches. Variegated branches are mosaics of all-green and all-white cells. Flowers can come from green, white, or variegated branches, but when crossed, it is the egg cell that determines branch color in the resulting plant. For example, if the egg cell comes from a flower on the white branch, regardless of the origin of the pollen, the resulting plant will have white branches, thus showing maternal inheritance. The variegated zygotes (bottom of Figure 4-30) are cytoplasmic mixtures of two chloroplast types, and, interestingly, these two types undergo cytoplasmic segregation during cell division, yielding the distinct green and white sectors that cause the variegation in the branches.

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Variant phenotypes caused by mutations in organelle DNA are generally inherited maternally.

© 1999 by W. H. Freeman and Company. All rights reserved.