Maternal Inheritance
Maternal inheritance is a type of uniparental inheritance in which all the progeny of a mating inherit the genotype and phenotype of the female parent. It is an extreme form of non-Mendelian inheritance and is seen as the failure of the progeny to display Mendelian segregation for certain characters. This bias in maternal genotype is established at, or soon after, the production of the zygote following mating, when the female sex cell (or gamete), containing the maternal genetic information in the nucleus, unites with the male gamete, containing information from the male. The nuclei of the male and female gametes fuse and the genetic information from mother and father is mixed. In the vast majority of species, the female gamete is physically larger than the male gamete and provides the cytoplasm for the developing embryo. This cytoplasm also contributes the organelles that are normally located there, as well as a number of proteins and messenger RNAs (mRNAs), encoded by the maternal nucleus prior to fusion. The organelles contain their own genetic information and the phenotypes controlled by their genes are said to exhibit maternal inheritance. In contrast, those phenotypes that are controlled by factors found in the zygote cytoplasm, but which are under the control of the maternal nucleus (such as the proteins and mRNAs) are said to express a maternal effect. An example of a maternal effect is the direction of coiling in the shells of pond snails or the patterning of the embryo in fruit flies.
True maternal inheritance results from the presence of DNA in organelles, such as mitochondria (in all species) and chloroplasts (in plants). This DNA is inherited independently of nuclear genes. Organelle DNA has been physically sequestered from nuclear DNA and its transcription, translation, and protein expression occurs within the same organelle compartment in which it resides. By contrast, nuclear DNA is expressed in the cell cytoplasm. Conditions in the organelle are different from those in the nucleus and organelle DNA, therefore, evolves at its own distinct rate. If inheritance is uniparental, there can be no recombination between parental genomes, and even in those cases where organelle genomes are inherited from both parents, recombination does not occur. Since organelle DNA has a different replication system from that of the nucleus, the mutation rate can be different. For example, mitochondrial DNA in mammals accumulates mutations more rapidly than nuclear DNA.
The classic study of maternal inheritance was performed in 1909 by the German plant geneticist Carl Correns (1864-1933). Correns was working on the four o'clock plant (Mirabilis jalapa), a variegated plant showing patches of green and white tissue. Some branches carry only green leaves while others carry only white leaves. Flowers can develop at different locations on the plant and can be intercrossed in a variety of different combinations by transferring the pollen from one flower to another. The leaves that emerge always corresponded to the color of the leaf of the female. For example, regardless of whether the male pollen is from a green, variegated, or white leaf, if the female flower comes from a region where the leaves are green, all the progeny leaves will be green. This is because differences in leaf color are due to the presence of either green or colorless chloroplasts that are never transmitted through the pollen of the male parent. All the organelle DNA that is found in the embryo after fertilization is from the female partner and determines the color of the leaves.
Knowledge of maternal inheritance has been applied in studies on human evolution. One consequence of maternal inheritance is that the sequence of mitochondrial DNA is more sensitive than nuclear DNA to reductions in the size of the breeding population. Comparisons of mitochondrial DNA sequences in a range of human populations allow evolutionary trees to be constructed. The divergence among human mitochondrial DNA spans 0.57%, and a tree can be constructed in which the mitochondrial variants diverged from a common (African) ancestor. The rate at which mammalian mitochondrial DNA accumulates mutations is 2-4% per million years. Such a rate would generate the observed divergence over an evolutionary period of 140 000-280 000 years. This implies that the human race is descended from a single female (the "mitochondrial Eve"), who lived in Africa around 200 000 years ago.
This is the complete article, containing 711 words
(approx. 2 pages at 300 words per page).
