MITOCHONDRIAL INHERITANCE (Maternal Inheritance)
 

 

Genes are part of chromosomes located in the nucleus of every cell. The chromosomes are made up of the chemical called DNA.

 

Another place where DNA is found is in very small compartments called mitochondria (mtDNA) which are found randomly scattered in the cytoplasm of the cell outside the nucleus.

 

Thus mitochondria contain genes too, although the mitochondrial DNA is one long

string of genes and is not arranged as chromosomes.

 

WHAT ARE MITOCHONDRIA?

Mitochondria are the little factories in each of the cells of the body that are responsible for making most of the body’s source of energy.
 

Body organs (especially the brain, heart, muscle, kidneys and liver) cannot function normally unless they are receiving a constant supply of energy. The energy is produced in the form of a chemical called ATP (adenosine triphosphate) which is used by the body to drive the various reactions essential for body functioning, growth and development.
 

A number of biochemical reactions which occur in an ordered sequence within the mitochondria are responsible for this process of ATP production. These reactions are under the control of special proteins called enzymes. The genes found within the mitochondria contain the information which codes for the production of some of these important enzymes.

 

WHAT ARE THE BIOCHEMICAL REACTIONS THAT OCCUR IN THE MITOCHONDRIA?

The biochemical processes which occur in the mitochondria and produce energy are known as the “mitochondrial respiratory chain”. This “chain” is made up of five components called Complex I, II, III, IV and V.  Each of these complexes are made up of a number of proteins. The message for the body to produce these proteins is contained in separate genes.
 

There are over 80 different genes needed to produce the components of the mitochondrial respiratory chain. Some of these genes are found in mitochondria rather than in the nucleus.

 

Mutations in any of these mitochondrial genes can result in biochemical problems due to absence or malfunctioning of the enzymes involved in the respiratory chain complexes. This leads to a reduction in the supply of A TP. This can have severe consequences, resulting in interference of body functions including any of the following, either in isolation or in various combinations

 

 EXAMPLES OF THE IMPACT OF FAULTY (MUTATED) MITOCHONDRIAL GENES

 

General: small stature and poor appetite

 

Central nervous system: developmental delay/intellectual

disability, progressive neurological deterioration, seizures, stroke-like episodes (often reversible), difficulty swallowing, visual difficulties and deafness

 

Skeletal and muscle: floppiness, weakness and exercise intolerance

 

Heart: heart failure (cardiomyopathy) and cardiac rhythm disorders

 

Kidney: abnormalities in kidney function

 

CAN MUTATIONS IN THE MITOCHONDRIAL GENES BE INHERITED?

The number of mitochondria in every cell of a horse’s body varies from a few to hundreds. All of these mitochondria, and therefore the DNA within the mitochondria, descend from the small number of mitochondria present in the original egg cell at the time of that horse’s conception. The sperm does not contribute any mitochondria to the baby.
 

Thus an individual’s mitochondria are only inherited from his or her mother. An abnormality in one of the mitochondrial genes can therefore be passed by the mother in her egg cells. As most of the mother’s egg cells carry the same mitochondrial mutation, the risk of this mother having another affected offspring with the mitochondrial disorder is high.  This pattern of inheritance is therefore referred to as maternal inheritance.

 

Super mt-DNA

Certain individuals in the Horse Breeding Industry believe that great stakes producing mares have some mutated special mt-DNA when coupled with large heart sized genes produce stakes class performance. (Historical mares in this class quoted in this category are Selene, Plucky Leige, Nogara and more recently Mrs Moss and Height of Fashion)