Genetic Selection of Horses

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Genetic Selection of Horses
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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

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What traits do horse breeders select for?

• Soundness
• Conformation
• Reproduction
• Athletic ability
• Speed
• Style (Hunter etc.)

• Color pattern
• Disposition/manners
• Performance

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Difficult to Measure Traits

• Conformation
• Disposition
• Athletic ability
• Intelligence

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GENETIC RELATIONSHIP TO OFFSPRING
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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

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Breeding Systems

• Outbreeding - mating crosses of horses less
  related than the average within a breed
• variability large
• heterozygosity increases
• increases genetic pool

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Outbreeding
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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

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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
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Effect of inbreeding

• Uncover undesirable recessive genes
• Reduced vigor
• Reduction in growth rate
• Increased mortality in offspring
• Reduced immune response of foals

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Performance Trait Selection

• How do we measure performance traits
• of wins/placings in shows
• dollars earned
• points earned/horses beaten
• race placings - speed

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Performance Testing

• 100 day test in warmbloods at stallion testing
  center
• Train and compete horses before breeding

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Progeny testing

• Train and compete offspring
• Only breed those animals with superior
  performance offspring

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Sib or family selection

• 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).
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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

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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

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Heritability Estimate
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Selection process

• Greater progress when keeping of selected
  traits to a minimum
• Low heritabilty est. traits increase success by
  controlling environment and management regimes

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Two basic types of genetic action

• Qualitative
• Particular trait is influenced by a single pair
  of genes (or maybe 2 or 3 pairs)

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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

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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

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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

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Horse GTG Idiogram

• E. caballus
• 64 chromosomes
• E. assinus
• 62 chromosomes
• Odd hybrids are sterile
• Mule
• Hinny
• zorse

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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

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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

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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

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Qualitative example

• HYPP - Hyperkalemic periodic paralysis
• 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.

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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.

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Qualitative exampleLETHAL 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

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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

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WHATS NEW?

• Juntional Epidermolysis Bullosa (JEB)
• Belgians
• Moderate-severe blistering of skin mouth
  sloughing of hooves in newborns

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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

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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

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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

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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.

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Exertional Rhabdomyolysis

• Most racing Quarter Horses, Thoroughbreds,
  Standardbreds and Arabians with tying-up suffer
  from a separate disease from PSSM called
  recurrent exertional rhabdomyolysis

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Hyperelastosis Cutis (HC/HERDA)

• Lack of adhesion within the dermis due to a
  collagen defect
• Recessive
• Poco Buenos bloodline