How Equine Genetic Diseases are Inherited
This article explains how genetic diseases are inherited when it comes to equine breeds. The so-called blueprints are present within the horse’s genes, and if viewed and studied under battery powered microscopes consist of deoxyribonucleic acid or DNA, a nucleic acid molecule in the form of a twisted double strand shaped as a double helix that is the major component of chromosomes and carries genetic information. There are two chains that make up this molecule, and contain various nucleotides adenine (A), cytosine (C), guanine (G) and thymine (T), which are clustered together into sets of three like ACG, CGG or TCA for starters. These groups of three are called codons that when stimulated, produces the code that dictates the production of amino acids.
Genetic anomalies arise when there is an error in the sequence of the nucleotides. Stephanie Valberg, DVM, PhD from the University of Minnesota explains that the problems are when there is a change in the sequence of letters or when a letter in the cluster is dropped. When the first letter of the chain of three letters is dropped, the cellular machinery does not recognize the sequence and the type of amino acid is not properly produced and is out of sync. This may result in the protein that is supposed to be coded by the mutated or altered gene is not correct or the protein may not be produced at all. The function of the much needed protein may be adjusted as well and this has a great effect on the animal’s survival.
Gus Cothran, PhD, from the Equine Genetics Lab at Texas A&M University states that the proteins are highly dependent on the cluster of three nucleotides to function. If the protein is not produced or does not work efficiently, this may lead to health dilemmas in the horse.
Mutation, a random change in a gene or chromosome resulting in a new trait or characteristic that can be inherited, can occur quite frequently when the DNA duplicates itself during the cell division process. The mutation can also result from DNA that is damaged from radiation or chemical exposure. Most of these mutations do little harm, but a transmutation that alters the critical function may be passed on to future offspring, potentially becoming a new genetic disease. You can view and observe how these mutations occur by using microscopes such as battery powered microscopes.
Most but not all horses who own a gene that is mutated or damaged show signs of disease or may not appear to be sick. As viewed under microscopes like battery powered microscopes, genes are normally found in pairs that perform the same function, like dictating the color of the eyes. But the dominant gene of the pair may alter the function of both it and the recessive gene. When a disease is a dominant one, only a copy of the dominant gene is needed for the disease to exhibit signs and symptoms. On the other hand, if the disease is present in the recessive gene, two copies of that gene is required to express the disease. For example, if the foal has been passed on the disease in a recessive gene from both parents, the disease will occur. If it is only passed on from one parent, the foal will just become a carrier and is likely not to exhibit the disease but instead pass it on to its future offspring.
ORIGINAL TEXT:
The blueprints for the structure of every life from, and the instructions for the activity of its every cell, are found within its genes, which are made up of DNA (deoxyribonucleic acid). The familiar double-helix-shaped molecule is made of two chains of nucleotides–adenine (A), cytosine (C), guanine (G) and thymine (T).
These nucleotides are grouped into sets of three (ACG, CGG, TCA, for example), called codons, that, when activated (expressed), ultimately code for the production of an amino acid.
The problems arrive when there is an error in the sequence of nucleotides.
“A change in the sequence of letters, or when one letter is dropped, can change the function of the gene,” says Stephanie Valberg, DVM, PhD, of the University of Minnesota. “If the first letter in the chain is dropped, then the cellular machinery reads the sequence out of sync affecting which type of amino acid is produced.”
As a result, the protein coded by the mutated gene may not have the correct amino acid sequence or the protein may not be produced at all. In either case, the function of the protein may be altered which can effect the animal’s survival.
“Proteins, particularly enzymes, depend on a three-dimensional shape to function and a change in the amino acid structure can change that,” says Gus Cothran, PhD, of the Equine Genetics Lab at Texas A&M University. “The protein may not work efficiently or protein production can stop entirely, leading to major problems.”
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Mutations–random changes in the nucleotide sequence–occur constantly when the DNA replicates itself during cell division (or when the DNA is damaged, by radiation, chemical exposure, etc.) Most do little if any harm, but a mutation that disrupts a critical function, and that can be passed on to descendants, will become a new genetic disease.
Not all horses who possess a mutated gene express clinical signs of disease. Genes occur in pairs that perform the same function (govern eye color, for example), but the activity of one may dominate (dominant gene) the activity of the other (recessive gene); a pair of genes may also be co-dominant, meaning that each contributes equally to the resulting trait. When a disease is dominant, only one copy of the dominant gene is necessary for disease expression. When a horse with a dominant trait is bred to a healthy horse there is a 50 percent chance of that offspring developing the disease.
If the mutated gene is recessive, two copies are required to express the disease. A horse with one copy of the mutated gene will be completely normal–he will be a “carrier” of the disease. But when he reproduces, there will be a 50-50 chance that he will pass the mutated gene on to his offspring. If the foal receives a recessive gene from both parents, then there will be no healthy dominant counterpart to take over that gene’s function, and the foal will experience the disease.
If a carrier mates with a noncarrier, each foal has a 50-50 chance of either becoming a new carrier or of inheriting two healthy genes. But, when two carriers mate, each has the potential to pass on the mutation, so the resulting foal has a 25 percent chance of inheriting only the two healthy genes, a 50 percent chance of becoming a new carrier of one mutated gene, and a 25 percent chance of getting both mutations and acquiring the disease.
To read more about equine genetic diseases, see “Genetic Tests” in the February 2007 issue of EQUUS magazine.Read more on this subject