Recent announcements of the newly discovered genetic disorders Hereditary Equine Regional Dermal Asthenia (HERDA) [formerly referred to with the less accurate term Hyperelastosis Cutis (HC)] and Glycogen Branching Enzyme Deficiency (GBED) draw attention to the importance of breeding management in horses. 

There are two modes of inheritance of genetic disorders, dominant genes and recessive genes
(Table 1).  With dominant genetic disorders, such as Hyperkalemic Periodic Paralysis (HYPP), only one copy of the defective gene is required for the horse to have the disorder and exhibit symptoms.  With recessive disorders, horses can be carriers, that have only one copy of the defective gene and do not suffer from its effects.  Horses expressing recessive disorders must have both copies of the defective gene, in other words, they must get one copy from each parent.  Therefore, both parents must be carriers or have the disorder.  HERDA and GBED are disorders with recessive transmission. 

Disorders with dominant transmission are rare and are (potentially) expressed in all of the affected individuals, but are easy to track in a population and to eliminate from breeding programs.  If horses with a dominant disorder are used in breeding programs, 50-100% of their offspring will also have (and express) the disorder.  There is a genetic test for HYPP, which makes it easy to diagnose. 

Genes for recessive disorders are relatively common in populations but are expressed only in horses with both copies of the defective gene, which means they are less harmful than dominant disorders to individual horses.  However, they are more difficult to track in a population and to eliminate from breeding programs.  They do not show up in populations until horses with identical close ancestors (which are the carriers) are mated.  All horses that have produced one carrier offspring are affected, must be carriers or have both copies of the mutated gene and should not be crossed with other carriers in the future. 

 

Table 1.  Characteristics of Equine Genetic Disorders
Characteristic	Dominant Disorder	Recessive Disorder
Incidence of gene in population	Relatively rare	More common
Expression in individuals	More common	Less common
Ease of tracking in population	Easy	Difficult
Copies of gene required for expression	1	2
Ease of elimination from population	Easy	Difficult
Carriers possible	No	Yes
Percent of offspring with disorder when heterozygous horse (with one normal and one defective gene) is crossed with normal, heterozygous and homozygous horses	50-75-100%	0-25-50%
Percent of offspring with disorder when homozgous horse (with both defective genes) is crossed with normal, heterozygous and homozygous horses	100-100-100%	0-50-100%
Examples in horse genetics (see explanations below)	HYPP, PSSM, RER	GBED, HERDA, OLWS, SCID

 

Beware of These Equine Genetic Disorders

GBED - Glycogen Branching Enzyme Deficiency – Recessive.  Found in some lines of Quarter Horses and related breeds.  Always fatal in utero or within 4 months of birth.  Results from lack of the enzyme allowing branching of glycogen (sugar units) in muscles and resulting lack of nutrients required for survival.  Prevent by not crossing two carrier horses.  A genetic test for GBED is in development by the University of California at Davis. 

Information:  http://academic-server.cvm.umn.edu/neuromuscularlab/Home.htm

HERDA - Hereditary Equine Regional Dermal Asthenia, Formerly called Hyperelastosis Cutis (HC) – Recessive.  Found in some offspring of the Quarter Horses stallion “Poco Bueno.”  Average life span is two-four years.  Skin layers separate due to collagen defect, and horses cannot be ridden.  Prevent by not breeding two carrier horses. 

Information:  http://www.vetmed.ucdavis.edu/ceh/RP03GEN0307.html

HYPP - Hyperkalemic Periodic Paralysis – Dominant.  Genetic mutation of sodium channel in muscle membranes that results in zero to severe muscle symptoms and possible death from heart failure.  All affected horses trace back to the Quarter Horse Stallion “Impressive.”  AQHA registered horses have a designation of N/N for normal, H/N for single gene and H/H for both mutated genes on the registration papers.  Prevent HYPP by testing all Impressive-bred horses and not breeding horses with H/N or H/H designation.  Management of affected horses is possible with medication from a veterinarian and/or carefully balanced low-potassium rations. 

Information:  HYPP

RER - Recurrent Exertional Rhabdomyolysis – Dominant.  Genetic form of tying-up or muscle damage and stiffness usually found in young, excitable Thoroughbred racing fillies.  Management is possible by reducing starch and adding fat and following carefully managed exercise program.  Diagnosis is possible with muscle biopsies. 

Information:  http://academic-server.cvm.umn.edu/neuromuscularlab/Home.htm

OLWS - Overo Lethal White Syndrome –- Recessive.  Disorder of Paint and Pinto horses resulting in pure white color and incomplete formation of the digestive tract.  Always fatal in utero or within hours of birth.  Prevent OWLS by not crossing two horses with Overo (one form of pinto spotting) coloring or breeding.  A test for the Overo gene is available from the American Paint Horse Association. 

Information:  www.apha.com. 

PSSM - Polysaccharide Storage Myopathy – Dominant.  Genetic form of tying-up with muscle damage and inability to move, usually found in Quarter Horses, Draft Horses and crossbreds.  Horses make and store abnormal muscle glycogen and cannot tolerate dietary starches and sugars.  Horses with PSSM can be maintained with low-starch and low-sugar rations and precise exercise protocols.  Diagnosis is possible with muscle biopsies. 

Information:  http://academic-server.cvm.umn.edu/neuromuscularlab/Home.htm

SCID – Severe Combined Immunodeficiency – Recessive.  Lack of normal immune function in Arabian Horses.  Usually fatal early in life.  Prevent by not breeding two carriers.  No genetic test at this time. 

Information:  http://www.vetgen.com/services.html#Equine

 

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