Diseases - Test 5 Flashcards
What does the term “genetic load” mean?
number of recessive disease causing genes carried by an individual
What does the term “carrier” refer to with respect to disease genes?
horse w/ single copy of recessive disease gene, which is inapparent from phenotype
Why might a partially dominant disease gene become common in a population of horses?
it may be linked to a desirable gene
What’s another reason partially dominant gene may become common in population of horses?
it is pleiotropic, having both beneficial and deleterious effects and selected by breeders
What is the mode of inheritance for HYPP?
partial dominant
What is the mode of inheritance for SCID?
recessive
HYPP appeared as a mutation in the stallion named Impressive. Why did HYPP subsequently become so common among Quarterhorses?
breeders selected for muscularity caused by this gene - horses w/ gene did well in halter classes
What was the negative aspect of the HYP gene that caused some breeders to want to control it?
horses with this gene could experience paralysis during exercise
The molecular basis for HYPP was discovered in 1992. What method was used to discover it?
candidate gene
Was a candidate gene used to discover SCID?
didn’t work initially when looked at gene associated with humans but somewhat related to mouse SCID
Which breeds of horse are affected by severe combined immunodeficiency disease?
Arabians
What are the symptoms of SCID?
absence of immune system; death from opportunistic infections by age 3
If you wanted information about diseases of livestock, which source would be most accurate and specific?
OMIA - Online Mendelian Inheritance in Animals… database created by Frank Nicholas in Australia
Why are some genetic diseases difficult to study?
- breeders do not continue to breed when they occur
- some have multiple genetic causes
- some are caused by more than one gene
- some have variable symptoms (poor phenotypes)
JEB1 and JEB2 occur in Belgian Draft horses and American Saddlebred horses, respectively. They are:
phenocopies
Founders are individuals who are first known carriers of a disease gene and the source for future generations. Which of the following diseases does not have a single, known founder?
ACAN dwarfism: 4 different alleles, no way of knowing who had the first mutation for them, we only know there are at least 4 of them
What is the mode of inheritance for HERDA?
recessive or partial dominant?
What is the mode of inheritance for dwarfism?
recessive
What is the mode of inheritance for JEB?
recessive
What are problems with family studies for disease?
- no one collects families
- no one repeats matings that produce disease
- no one likes to find hereditary problems in blood stock
What does HYPP mean?
Hyperkalemic Periodic Paralysis
What does HYPP do?
- defect of sodium pump in muscle
- failure to exchange K+ and Na+
- build up of potassium
- paralysis: failure of muscle contraction response
- response varies from asymptomatic to severe
- influenced by diet: ex. alfalfa and molasses have high K+
What does SCID mean?
Sever combined Immunodeficiency Disease
What is the carrier rate for SCID?
in 1996, it was 8.4%
What does MH mean?
malignant hyperthermia
What kind or trait is MH?
dominant trait impacted by management
What is the effect of MH?
usually inapparent in horses unless in surgery,
halothane anesthesia can be lethal
How was MH found?
candidate gene for equine MH from human studies (RYR1)
What does JEB mean?
Junctional Epidermolysis Bullosa
What is JEB?
- disease of basement membrane of skin that causes sloughing of skin
- shortly after birth skin sloughs off following handling
Where is defect found for JEB1?
defect of LAMC2 found in Belgian horses
Where is defect found for JEB2?
defect of LAMA3 in American Saddlebred horses
What does HERDA mean?
Hereditary Equine Regional Dermal Asthenia
What is HERDA?
a recessive defect of skin that causes sloughing of skin under saddle; need two copies to be affected
- defect in gene PPIB
In what horses is HERDA found?
Cutting horses (Quarter horses)
What causes dwarfism?
- 4 variants in ACAN gene of Shetland ponies and miniature horses that are recessive causes of dwarfism + embryonic loss
- recessive SHOX defect on X/Y chromosomes of Shetland ponies and miniature horses causes recessive limb shortening
- recessive B4GALT7 defect in Friesian horses causes limb shortening
What is the carrier rate for ACAN variants?
26.2%
What is deleterious mutation load?
- recessive deleterious genes that exist in individuals that will cause disease
- usually very rare… spontaneous mutation but no effect on phenotype
- only appear when individual inherits two copies
