Title: Genetic Selection of Horses
1Genetic Selection of Horses
2Introduction
- 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
3What traits do horse breeders select for?
- Soundness
- Conformation
- Reproduction
- Athletic ability
- Speed
- Style (Hunter etc.)
- Color pattern
- Disposition/manners
- Performance
4Difficult to Measure Traits
- Conformation
- Disposition
- Athletic ability
- Intelligence
5GENETIC RELATIONSHIP TO OFFSPRING
6Breeding 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
7Breeding Systems
- Outbreeding - mating crosses of horses less
related than the average within a breed - variability large
- heterozygosity increases
- increases genetic pool
8Outbreeding
9Breeding 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
10 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
11Effect of inbreeding
- Uncover undesirable recessive genes
- Reduced vigor
- Reduction in growth rate
- Increased mortality in offspring
- Reduced immune response of foals
12Performance Trait Selection
- How do we measure performance traits
- of wins/placings in shows
- dollars earned
- points earned/horses beaten
- race placings - speed
13Performance Testing
- 100 day test in warmbloods at stallion testing
center - Train and compete horses before breeding
14Progeny testing
- Train and compete offspring
- Only breed those animals with superior
performance offspring
15Sib 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).
16Limitations 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
17Heritability 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
18Heritability Estimate
19Selection process
- Greater progress when keeping of selected
traits to a minimum - Low heritabilty est. traits increase success by
controlling environment and management regimes
20Two basic types of genetic action
- Qualitative
- Particular trait is influenced by a single pair
of genes (or maybe 2 or 3 pairs)
21Two 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
22Type 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
23Types 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
24Horse GTG Idiogram
- E. caballus
- 64 chromosomes
- E. assinus
- 62 chromosomes
- Odd hybrids are sterile
- Mule
- Hinny
- zorse
25Parentage 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
26Parentage 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
27Qualitative 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
28Qualitative 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.
29Qualitative 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.
30Qualitative 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
31Quantitative 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
32WHATS NEW?
- Juntional Epidermolysis Bullosa (JEB)
- Belgians
- Moderate-severe blistering of skin mouth
sloughing of hooves in newborns
33Glycogen 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
34Glycogen 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
35Polysaccharide 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
36PSSM
- 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.
37Exertional Rhabdomyolysis
- Most racing Quarter Horses, Thoroughbreds,
Standardbreds and Arabians with tying-up suffer
from a separate disease from PSSM called
recurrent exertional rhabdomyolysis
38Hyperelastosis Cutis (HC/HERDA)
- Lack of adhesion within the dermis due to a
collagen defect - Recessive
- Poco Buenos bloodline