Genetic Selection of Horses
Introduction
• Genetics are not as utilized in the horse
industry as in livestock species who select for
size, length, weight, efficiency of or maximal
production of livestock
• Horse breeders look more at pedigree, eye
catching traits, and subjective criteria
What traits do horse breeders
select for?
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Soundness
Conformation
Reproduction
Athletic ability
– Speed
– Style (Hunter etc.)
• Color pattern
• Disposition/manners
• Performance
Difficult to Measure Traits
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•
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Conformation
Disposition
Athletic ability
Intelligence
GENETIC RELATIONSHIP TO
OFFSPRING
RELATIONSHIP
DIRECT
Sire or Dam
Grand sire or dam
Great grand sire or dam
COLLATERAL
Full-sib
3/4 Brother or sister
Half-sib
First cousin
% RELATIONSHIP
50.0
25.0
12.5
50.0
37.5
25.0
12.5
Breeding Systems
• Crossbreeding - breeding different breeds
together
– major heterosis - variable results
– breed registry book:
• closed - AHR and Half-Arabian
• open - ApHC and APHA to QH
– AQHA, ApHC, APHA to TB
Breeding Systems
• Outbreeding - mating crosses of horses
less related than the average within a
breed
– variability large
– heterozygosity increases
– increases genetic “pool”
Outbreeding
Native Dancer
Raise A Native
Lines of Power
Raise You
Bold Ruler
Exotic Garden
POLITICAL
WHIT
The Garden
Club
Matsadoon
Doonesbury
Political
Parody
Vaguely Nice
Roberto
Urakawa
Miss
Devereux
Breeding Systems
• Inbreeding – breeding horses more related than the breed
average
• increases homozygosity
• increases the predominance of recessive diseases
• uniformity of offspring increases
• Linebreeding - increases the relationship
to some admired ancestor
– relationships within a pedigree are additive
Three Bars
Zippo Pat Bars
Leo Pat
I Inspire
Big Step
Teeny Step
Teeny Brown
CK Zippos Bar Bee
Zippo Pat Bars
Zippo Bar Pine
Dollie Pine
Zip Bar Bee
Double Bar Leo
Money Bee Honey
Chetterette Bar
Effect of inbreeding
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•
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Uncover undesirable recessive genes
Reduced vigor
Reduction in growth rate
Increased mortality in offspring
Reduced immune response of foals
Performance Trait Selection
• How do we measure performance traits
– # of wins/placings in shows
– dollars earned
– points earned/horses beaten
– race placings - speed
Performance Testing
• 100 day test in warmbloods at stallion testing
center
• Train and compete horses before breeding
Progeny testing
• Train and compete offspring
• Only breed those animals with superior
performance offspring
Sib or family selection
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•
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Relies on performance of collateral relatives
Performance data in the pedigree
Often seen in sales catalog pages and reference stallions
Look for black type
1st dam
BRAVE RAJ, by Rajab. 6 wins in 9 starts at 2, $933,650, champion 2-year-old filly in U.S.,
Breeders' Cup Juvenile Fillies S. [G1], Del Mar Debutante S. [G2], Sorrento S. [G3], Florida
Stallion/My Dear Girl S. [LR] (CRC, $240,000), Florida Stallion/Susan's Girl S. [LR] (CRC,
$45,000), 2nd Junior Miss S. [L] (DMR, $10,000). Dam of 14 foals, 11 to race, 10 winners-BRAVO BULL (c. by Holy Bull). Black type winner, see below.
RUSSIAN TANGO (f. by Nijinsky II). 4 wins at 3 and 4, $92,390, The Very One H. [L]
(GP, $30,000). Dam of-EUROSILVER (c. by Unbridled's Song). 4 wins, 2 to 4, 2005, $622,310, Lane's End
Breeders' Futurity [G2] (KEE, $248,000), Skip Away H. [G3] (GP, $60,000), 2nd Stephen
Foster H. [G1] (CD, $165,600), Swale S. [G3] (GP, $30,000), Perryville S. [L] (KEE,
$22,520).
Limitations of stallion indexes
• Average of all offspring is unknown
– only superior animals compete
• Mare contributions hard to quantify
– stallion managers screen superiority
• Points, $$’s, and purse size influences
• Miles and resources give advantage
– Honor roll vs world champion
Heritability Estimate
• Definition
– Percentage of a horse's expressed trait (phenotype)
that is due to genetics
– Due to genetics indicates the probability of trait
being passed on from one generation to the next
– Specifically, ability to select horses to mate based on
superior performance for the trait and to predict the
improvement in the offspring
Heritability Estimate
Trait
Due to Genetics
Due to
(%)
Environment (%)
Height at withers
45-50
50-55
Body weight
25-30
70-75
Body length
35-40
60-65
Pulling Power
20-30
70-80
Running speed
35-40
60-65
Walking speed
40-45
55-60
Trotting speed
35-45
55-65
Temperament
25-30
70-75
Selection process
• Greater progress when
keeping # of selected
traits to a minimum
• Low heritabilty est.
traits: increase success
by controlling
environment and
management regimes
Two basic types of genetic action
• Qualitative
– Particular trait is influenced by a single pair of
genes (or maybe 2 or 3 pairs)
Two basic types of genetic action
• Quantitative
– Trait that is influenced by numerous different
genes
– Effects of many genes are added together to
produce trait
– Each gene only has small effect of trait
– Most traits are quantitative
Type of Qualitative
• Dominance: ability of gene to mask or
cover up its recessive allele
– Only one dominant gene is required to
display a particular trait
– Two recessive genes are required for a
recessive trait to be exhibited
Types of Qualitative
• Co-dominance
– Gene action results in an intermediate state between
two parents example: blood type
– Each blood type is different, known, and indicates
genotype
Horse GTG Idiogram
• E. caballus
– 64 chromosomes
• E. assinus
– 62 chromosomes
• Odd hybrids are sterile
– Mule
– Hinny
– zorse
Parentage Testing
• Blood type group - AB on RBC surface
– electrophoresis of 8 proteins - very specific
– used to detect NI (neonatal
isoerthyyhtolysis) - like Rh factor in
humans - dam makes AB against foal
blood type
– blood type is example of co-dominance
Parentage Testing
• DNA Testing - uses microsatalite allele
length variants - uses hair follicles
• Polymerase Chain Reactions (PCR) & tag
with florescent dye and sized by gel
electrophoresis
• Breeds all contract labs - need dam, sire,
and foal
Qualitative example
COMBINED IMMUNE DEFICIENCY (CID)
• Deficiency of B & T lymphocytes - susceptible
to adenoviral pneumonia
– Arabian & part-Arabian horses
– Autosomal recessive - homozygous
– cid cid - diseased
– CID cid - carrier; CID CID – normal
• Normal foal at birth.
• Illness 2-5 months of age.
• Die of infection
Qualitative example
• HYPP - Hyperkalemic periodic paralysis
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sporadic attacks of muscle tremors, collapse
hyperexcitable muscle from voltage-gated K channel
Gene mutation in Impressive (QH)
Autosomal dominant - not sex linked
H/H and H/h are affected and hh - normal
Industry refers to hh as N/N
Hair follicle test
• AQHA - Foals born January 1, 2007, and testing
positive for the H/H status of HyPP, will no longer
be eligible for registry.
Qualitative example
Color related allele - partial dominance
• Ww - White
• WW: Lethal
• Ww: Horse typically lacks pigment in skin, hair
and eyes and appears to be white.
• ww: Horse is fully pigmented.
Qualitative example
“LETHAL WHITE IN OVERO PAINT HORSES”
• Colon is formed but its nerve supply is missing - gut is paralyzed.
• Atresia Coli - Sections of colon are missing.
• Lethal White Overo (LWO) - not selective to the paint color overo
pattern
• extreme co-dominant - not really dominance/recessive
• use allele specific PCR hair test
• if O is sequence for overo gene, then
– OO is lethal
– ON is paint color pattern
– NN is solid color “breeding stock”
•
colored x colored gives 25% lethal
Quantitative
example
• Racing
– Affected by: size, length of leg, coordination,
efficiency of heart, lungs, muscles, mental traits
that affect the desire and determination of horse
• 2 components involved
– Heredity: genetic component
– Environment: nutrition, training
• Combination of many genes
WHAT’S NEW?
• Juntional Epidermolysis Bullosa (JEB)
– Belgians
– Moderate-severe blistering of skin & mouth &
sloughing of hooves in newborns
Glycogen Branching Enzyme
Deficiency (GBED)
• Glycogen branching enzyme - protein
necessary to build glycogen
• Mutation in the GBE gene on chromosome
26
• Autosomal recessive
• Quarter Horse related breeds
• Fatal in all cases by time foals reach 8 wks
of age
Glycogen Branching Enzyme
Deficiency (GBED)
Biopsies from normal (left) and GBED-affected (right) horses stained
with PAS.
•Abortion or still birth of a foal
•Weakness, low body temperature at birth.
•Sudden death on pasture from the heart stopping or from seizures
•High respiratory rate & weakness of the muscles used to breathe
•Contracted tendons in all four legs
Polysaccharide Storage Myopathy
(PSSM)
• Muscle disease in Quarter Horses, Paint
Horses & Appaloosas.
• Another form of PSSM occurs in Draft,
Draft crossbreds, warmbloods.
• Enhanced insulin sensitivity &
accumulation of glycogen
• Severe muscle damage
PSSM
•Signs of tying-up
•Muscle stiffness, sweating & reluctance to move
•First noticed in horses when they are put into training or after a lay-up
period
•Episodes begin after very light exercise such as 10-20 min of walking and
trotting.
A normal biopsy (left) and a biopsy from a horse with PSSM (right)
stained with PAS. Note the lack of a uniform texture in the PSSM
biopsy. The darker areas in the PSSM biopsy indicate the
accumulation of excess glycogen and abnormal polysaccharide.
Exertional Rhabdomyolysis
• Most racing Quarter Horses, Thoroughbreds,
Standardbreds and Arabians with tying-up
suffer from a separate disease from PSSM
called recurrent exertional rhabdomyolysis
Hyperelastosis Cutis (HC/HERDA)
• Lack of adhesion within
the dermis due to a
collagen defect
• Recessive
• Poco Bueno’s
bloodline