(version 22 August 2005)
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Genes located outside of the nucleus (i.e., not on the major chromosomes)
Also referred to as cytoplasmic or extrachromsomal inheritance
mtDNA and cpDNA are almost always uniparentially inherited, with only one sex (typically the female) transmitting the genomes to their offspring.
The cytoplasm contributed by the female contain several components
Segregation at extranuclear elements occurs by different mechanisms than nuclear chromosomes. Hence, following a mitotic cell division, we can get out two different extranuclear cell types
Variegated patterns in plants
Classical data from the four-o'clock plant by Correns (1909)
Three types of branches observed:
By using flowers on these three types of branches, crosses between the three types of parents can be made, and the following data were observed
The punch line: Offspring phenotype is are entirely determined by the female parent:
This sort of pattern is know as maternal inheritance, where the offspring phenotype is entirely a function of the maternal genotype.
The four-o'clock data is explained by two things
poky mutants in the fungus Neurospora
Mary Mitchell (1952) isolated a series of slow-growing mutants from the fungus Neurospora that she called poky. Poky mutants have abnormal amounts of cytochromes, lacking cytochromes a and b and showing an excess of cytochrome c.
In fungi, one can construct crosses such that one parent contributes the vast bulk of the cytoplasm, with the cytoplasm-contributing parent being referred to as the female parent.
Poky is due to a defect in the mtDNA. As shown in the above figures,
shell coiling in snails
Not all maternal effects are due to extranuclear genes. As mentioned above, the material cytoplasm in multicellular organisms contains most of the proteins and mRNAs expressed in early development.
An especially interesting example of a materal effect due to the expression of maternal genes in her offspring is the inheritance of left vs. right shell coiling in the snail Linnaea, which was worked out by Sturtevant (1923)
As shown below, the direction of coiling is entirely determined by the genotype (not the phenotype) of the mother.
The explanation is that this gene determines the initial cleavage pattern in the developing embryo. Since this gene is expressed when the mother is provisioning her eggs, it simply depends on her genotype, not her current shell phenotype. The s + allele is dominant to s, so that only ss females result in left-handed coiled offspring.
Each cell contains tens to thousands of mitochondria, each of which it turn contains tens to hundreds of individual mtDNA genomes.
The mtDNA genome encodes only a very small fraction of the proteins required to make up the mitochondria. Hence, essentially all of the components of the mitochondria are encoded by nuclear genes.
the Human mtDNA is about 17,000 base pairs, and contains
As is shown on the human mtDNA map, several human diseases have been localized in the mtDNA genome
All sequenced mtDNAs have about the same number of genes. However, the mtDNA size for the different species vary greatly in size
As with mtDNA,
The cpDNA contains about 10 times as many genes as the mtDNA.
Typical size 120-220 kb
Typical number of genes is about 140, broken down as follows: