In general, mitochondria are responsible for the energy production in cells and specify the formation of some key components of the energy-producing system. Mitochondria provides about 90% of the energy that cells, and thus tissues, organs and the body as a whole needs to function. The primary function of mitochondria is to convert food into energy. They convert the food we eat into basic chemicals that the cell can use. Hence, they are known as the cells powerhouse.

How mitochondria are inherited is simple. When an egg is fertilized, mitochondrial DNA is passed on to the offspring. Sperm passes on almost no mitochondria. There is some seepage but most commentators do not appear to attach any significance to this and from an inheritance point of view, MtDNA is considered to pass down the female line. The female passes the MtDNA to both sons and daughters, but only daughters can pass it to the next generation. This pattern is different from sex linkage and similar to Y inheritance father to son only except mitochondrial can go to son also but the son does not pass this on further. The importance to breeders is if the genetic material contained in mitochondria has a significant impact on race performance it is passed down the bottom line of the pedigree only. This is where the cells metabolism is found. 

The genetic impact of this DNA is known mostly through the adverse consequences when mutations arise leading to diseases of energy production in cells. These mitochondrial diseases are most often seen in neurology clinics. Such diseases are rare and hardly ever described in animal systems (since the affected animals would be regarded as runts or so sick they would never survive in the wild). Breeders would also cull very quickly any affected offspring that survived.

Mitochondria are also involved in ageing, to the extent that it is now recognized that mutations in mitochondrial DNA do accumulate in ageing organisms, including humans and other mammals. At the same time, mitochondrial energy production declines in many tissues (particularly muscle, heart and brain) during the ageing process.

MtDNA makes up 0.0006% of the nuclear genome but amounts to 1% of the total mass of cellular DNA. It is a very significant part of the genome. It is present in every cell. 

Some Mitochondrial DNA Diseases:

Alzheimer's-progressive loss of cognitive capacity

CPEO  -eye paralysis Diabetes mellitus-high blood glucose

Dystonia -abnormal movements involving muscular rigidity, often associated with brain degeneration

KSS-CPEO  combined with such disorders as retinal deterioration, heart disease, hearing loss, and diabetes and kidney failure

Leigh's Syndrome -progressive loss of motor and verbal skills and degeneration of the basal ganglia (potentially lethal child disease)

LHON -permanent or temporary blindness stemming from damage to optic nerve MELAS-dysfunction of brain tissue

MERRF- may involve hearing loss and dementia Mitochondrial myopathy- deterioration of muscle

NARP -loss of muscle strength and coordination

Pearson's Syndrome -childhood bone marrow dysfunction Mitochondrial disorders are maternally inherited and generally affect many organs simultaneously and become progressively worse.

You will see from the sorts of disease though that these problems are extremely unlikely to be important to breeders as any individuals in the animal kingdom are unlikely to survive. Even if they did, breeders would soon cull them.

Why do these diseases often occur as part of the aging process? There are two speculations on this.

1.The first is that molecules bearing DNA deletions, being smaller, replicate faster and so the cells become enriched with the mutant DNA.

2.The second relates to the organization of muscle tissue. Each fiber consists of many merged muscle cells and contains multiple nuclei. Various findings imply that when a nucleus detects a deficient in its area, the nucleus attempts to compensate for the power shortage by triggering the replication of any mt in its area. Unfortunately, this promotes the replication of the very mt that are causing the problem and further aggravates the problem.

The origin of the deletions has puzzled scientists as even though these disorders are passed from generation to generation, deleted mt is rarely inherited. Possible explanation is the embryo containing the deleted mt dies. The heteroplasmic state is also short lived and descendants of a heteroplasmic mother become homoplasmic after a few generations. This is because oogonia contain a smaller number of mitochondria (200) than other cell types, which creates a population bottleneck that reduces diversity.

Mitochondria are considered to have a relatively high mutation  rate. I am referring here to mutations that may be inherited and not somatic mutations that occur throughout the lifetime of any individual. I am unsure as whether this happens over a few generations or 100's of generations. Compared to the genes carried on the other chromosomes the mutation rate in mitochondria is considered high. It appears that the rate of mutation is relatively constant and seems to occur every 2,000 to 3,000 years. 

Global migrations can be reconstructed through mitochondrial DNA analyses. Scientist by grouping differences in sequences can establish the pattern of global migration. African populations are oldest because there is a greater variation in the sequences. Asians, Europeans and Native North Americans display progressively less variation in the sequences and these areas were populated later. There are though recent studies that throw doubt on this theory being entirely correct and like anything to do with genetics and inheritance caution should be your constant companion. So far, mitochondrial inheritance sounds very simple. However, mitochondrial disorders do not show regular Mendelian patterns of inheritance. The patterns can be complex.

In each cell, there are hundreds of mitochondria (a single mitochondrion contains several loops of DNA, each of which includes 37 identified genes involved in energy production). Researchers have found in investigating diseases that have their source in mitochondria some interesting things happen. The resulting symptoms are more unpredictable than those caused by nuclear genetic mutations (these arise with the fertilization of an egg by sperm. The single-cell embryo emerging ends up with a single nucleus containing matching sets of chromosomes, one set from the mother and one from the father. The cell then replicates repeatedly to result in the child. Every cell of the body then carries an identical set of genes and identical mutations). This does not happen with mitochondria. 

Females being the bearers of secrets are much more complex. Each and all body cells carry not one but hundreds of mitochondria, and every mitochondrion can contain several mitochondrial DNA molecules. Although when this material replicates each daughter cell (the replicated cell) gets approximately an equal amount, the original cell does not regulate which specific mitochondria go to each daughter cell. The result is if an egg contains a mutation  in some fraction of its MtDNA (termed "heteroplasmy") one daughter cell may inherit a larger proportion containing the mutant DNA and another cell a larger proportion of the normal DNA. The result is a child can have cells carrying both normal and mutant MtDNA.

The eggs from a heteroplasmic mother can differ in their percentages of mutant DNA. The result is the children can differ markedly in the extent and distribution of the mutant molecules and even the symptoms they display. Children of homoplasmic mutation , all display the same symptoms.

Another confusing issue with mitochondria has to do with mutation . What has been stated above is correct. However, MtDNA mutations can also form in tissues throughout life. This somatic form of mutation use to really confuse me and from a breeding point of view is best ignored (refer to this again when dealing with older stallions).  MtDNA mutations generally become obvious as part of the aging process (experiments with rats show that by reducing free radicals, Vitamin C and restricted calorie diet the aging process caused by MtDNA can be slowed up markedly.

The following should really be in part 11 as it is speculation but will include the comments here, as I tend to ignore this material. I do take note though when I see pedigrees with a duplication of the bottom line of the mare somewhere else in the pedigree of an individual. To use and example most will be familiar with this happens in Might And Power 's pedigree. Not sure, whether this has any significance but take the attitude that if it is there it could be a bonus so toss it in. Bit like making fruit salad. I speculate that Mitochondria may influence how genes are expressed and therefore take notice of parts of the pedigree even though I know that this material cannot be inherited from that position. This material certainly affects the functioning of cells as it is how the cells derive their energy supply that they need to function. I include these as bonus elements just in case it influences the other genes that may be inherited in a non-sex linked manner and may be something that helps decide what combination of genes work best.

Whether this is what actual happens who knows but there seems to be something at work in pedigrees that favour reintroduction of different strains of the same female family. It should do no harm by getting a similar strain of genes that has worked well before when powered by the same MtDNA.

From an inheritance point of view the bottom line is all offspring of a mare inherit mitochondria but no offspring of a stallion can.