Maternal Effects

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Structure of Organelle Genomes

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Variegation in four o'clock plant and maternal inheritance

The classic study of maternal inheritance was performed by Correns on the four o'clock plant. This plant can have either green, variegated (white and green) or white leaves. Flower structures can develop at different locations on the plant and the flower color corresponds to the leaf color. When Correns crossed the different colored flowers from different locations on the female plant with pollen obtained from flowers of the three different colors, the progeny that resulted from the cross always exhibited the color of the leaf of the female. That is, regardless of whether the pollen was from a leaf that was green, variegated or white. If the female flower came from a region where the leaves where green, all the progeny were green. Similar results were seen when the female was from a region on the plant where the leaves were either variegated or white. In comparison to traits controlled by maternal effects, those traits controlled by maternal inheritance, the female phenotype is always expressed in its offspring.

Female Male Progeny Phenotype
Green Green, Variegated or White Green
Variegated Green, Variegated or White Variegated
White Green, Variegated or White White

The results can be explained in the following manner. All of the organelle DNA that is found in an embryo is from the female. The egg cell is many times larger than the pollen cells, and contain both mitochondria and chloroplasts. Pollen is small and is essentially devoid of organelles, and thus organelle DNA. So any trait that is encoded by the organelle DNA will be contributed by the female. In the case of the four o'clock plant, the different colors of the leaves is a result of the presence or absence of chlorophyll in the chloroplast, a trait that can be controlled by the chloroplast DNA. Thus, green shoots contain chloroplasts that have chlorophyll, the chloroplasts in the white shoots contain no chlorophyll, and the variegated shoots contain some chloroplasts with chlorophyll and some without chlorophyll. Thus, depending upon the location in the plant where the flower comes from, the egg can have chloroplast with chlorophyll, without chlorophyll or a mixture of the two types of chloroplasts. This is the biological basis of maternal inheritance.

Copyright © 1997. Phillip McClean