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Genetics
Phenotype/Genotype
• Phenotype is what an animal looks like
• Phenotype = Genetics + Environment
(+GxE interactions)
• Genotype = the genetic makeup of the
organism
Discovery of Genetics
• Gregor Mendel
• Ahead of that time there was no good
concept of transmission of genetic
information from one generation to the next.
Gregor Mendel
• Austrian Monk lived 1823-1884
• Presented his observations and experiments
on pea plants in 1865
• Discoveries lay unnoticed ~20 years until
others independently found the same thing
• He found traits were controlled by discrete
“factors” (genes)
Cell Theory of Inheritance
• All plants & animals are made of small
building blocks called cells
• Cells composed of:
– cell wall
– nucleus
– cytoplasm
• All cells originate from other cells
Cell Structure
Golgi Complex
Nucleus
Ribosome
Endoplasmic Reticulum
Vacuole
Lysosome
Unit of Inheritance
• Gene
• Genes are particular parts of DNA
• Contained in the NUCLEUS
DNA
• Deoxyribonucleic
Acid
• Contains the genetic
code by the
arrangement of 4 base
pairs. Up to 600/gene
• Structure of DNA by
Watson & Crick won
Nobel prize
DNA
• Made up of 4
nucleotides and
deoxyribose
• Forms genes
• Genes reside on
chromosomes
Chromosomes
• Made up of DNA
• Contain many genes
on each chromosome
• Not always visibile,
only when they coil up
• Occur in pairs in
somatic cells
Sex
• 1 pair of chromosomes • Mammals
determines sex
– Female = XX
– Male = XY
• Other traits on that
chromosome will be
“sex linked” traits
• (different
nomenclature for
poultry)
Some Terms
• Homozygous
– SAME
• Heterozygous
– DIFFERENT
More Terms
• Homologous = “member of pair”
• Dipoid = 2n number of chromosomes
• Haploid = 1n number of chromosomes
More more terms
• Dominance = gene always expressed
• Recessive = gene only expressed if not masked
• Codominant or Lack of Dominance = both
homologous genes expressed
Angus - Black is dominant, Red is recessive
Shorthorn - Red, White, No Dominance, All patterns, Roan
More more more terms
• Locus = Location on the chromosome of a
gene
• Allele = alternate genes that occupy
corresponding sites on homologous
chromosomes
– like black and red for angus cattle
Even MORE Terms
• Kinds of cell division
– Mitosis
• The way cells divide in somatic cells
• Results in diploid # of chromosomes
– Meiosis
• Cell division in sex cells (ova, sperm)
• Results in haploid # of chromosomes
Mitosis
1- Interphase
4- Anaphase
3- Metaphase
2- Prophase
5- Telophase
Meiosis
• Reduction division
• Occurs only in gametes (sex cells)
• Results in 1/2 the # of chromosomes
– (haploid number)
– 1 of each pair of homologous chromosomes
No. of Chromosomes by Species
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Horse
Donkey
Mule
Swine
Sheep
Cattle
Man
64
62
63
38
54
60
46
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Mink
Dog
Lion
Domestic cat
Bengal tiger
Chicken
30
78
38
38
38
78
Mendellian Genetics
• Explains the segregation and recombination
of genes
• Understandable for a small number of traits
at a time
• Understandable for traits controlled by 1 or
a few genes
• MOST Productivity traits = many genes
Abnormalities
• Mutation
– Accidental change in the structure of a gene
– Occur with low frequency randomly or from
radiation, chemicals, drugs, etc.
Mutation types- Crossing Over
Mutation types - Deletion
Mutation types - Duplication
Mutation types - Insertion
Are Mutations Good or Bad?
• Usually BAD
• Sometimes NO EFFECT
• Sometimes GOOD
– Polled condition in hereford cattle
Prediction
• When traits are controlled by single gene
pairs, predicting phenotype from genotype
is possible if we know the type of gene
action!
– Dominance
– Recessive
– Codominance
• More useful is predicting the GENOTYPE
from what we see of the animals
(phenotype)
– We can make matings and observe the outcome
– ONLY finds Statistical Probability in some
cases
• View now the genetic animations for
determining the combinations possible!
Livestock Improvement
• Most economically important traits involve
several or many genes
– Growth
• depends on appetite, gut capacity, metabolism rate
– etc etc etc
– Milk production
• depends on mammary development, cow size,
appetite, blood supply,
– etc etc etc etc
Therefore -- Population Genetics
• Goal is to select animals with many good
genes
• Remember P = G + E
– So to compare animals, keep the Environment
the same
Rules for Maximum Genetic
Improvement
• Have maximum genetic variation
• Spend selection efforts on traits largely
influenced by heredity
• Observe (measure) accurately the traits
carried by the animal
• Use the selected animal(s) most effectively
1. Have maximum genetic variation
• Uniformity may be good, but limits genetic
progress
• Breeding herds exist to provide best
genetics for future generations (and
improve)
2. Spend selection efforts on traits
largely influenced by heredity
• Heritibility h2
• The proportion of variation that can be
expected to be transmitted to the next
generation
• The relative importance of heredity in
influencing certain traits
• Heritability refers to TRAITS not the animal
Heritability estimates
• No. of young weaned
• % lean cuts
• Rate of gain
Cattle
10-15
40-50
50-55
Swine
10-15
30-40
25-30
Level of Heritability
• Low (5-15%)
– Reproductive traits
– Health
• Medium (15-40%)
– Conformation score (dairy, beef 25%)
– Many production characteristics
• High (40%+)
– Carcass characteristics
– Growth rate (cattle, sheep)
– Mature weight
How much progress can we
make?
• Depends on how much better the parents are
than the average of the population.
• Two parents, each has ½ the influence
• Depends on the heritability of the trait
• Progress = selection differential * h2
Selection differential
• How much better are the parents than the
average of the population they are selected
from
Example
Say herd population is 18,000 lbs of milk
– Choose a bull with a milking potential of
22,000 lbs of milk
– Choose cows with 20,000 lbs milking potential
– Bull
Cow
22,000
20,000
-18,000
18,000
------------------4,000
2,000
½ the genetics comes from bull, ½ from cow
So you can have ½ of 4000 and ½ of 2000
Example
½ the genetics comes from bull, ½ from cow
So you can have ½ of 4000 and ½ of 2000
(4000 + 2000) / 2 = 6000/2 = 3000
Multiply the Selection Differential (3000) by h2
H2 for milk production is 0.25
3000 X 0.25 = 750 lbs of improvement
Add that to the herd avg: 18,000 + 750 = 18,750
Which is the avg of production in the
replacements.
Example
• If the replacements = 10% of the herd,
• What is the new herd average?
– 90% of herd still averages 18,000
– 10% of herd averages 18,750
– (18,000)(.90) + (18,750)(.10) = 18,075
• If we replace 20% of the herd
– (18,000)(.80) + (18,750)(.20) = 18, 150
• As you can see, progress is slow
• So you must continue to strive to make
progress as steadily as you can
If you only selected the bull
• The selection differential on the bulls side is
the same (22000 – 18000 = 4000)
• Sel.Diff. On the cow side is 0
• 4000 / 2 = 2000
• (2000 X .25) = 500 which is improvement
• Add 500 to herd average
• (500 + 18000) = 18,500
Let’s do another example
• Suppose a swine herd average is 1.2 inches of
backfat
– Select a boar with 0.8 inches, gilts with 1.0 in.
(1.2 – 0.8) = 0.4
(1.0 – 0.8) = 0.2
(0.4 + 0.2) / 2 = 0.3
The offspring are expected to be 0.3 better
1.2 inches – 0.3 inches = 0.9 avg of next generation
What influences how much genetic
progress you can make?
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Amount of genetic variation
Heritability
Accuracy of measurement of information
Extent of use of selected animal
How extensively you can make use
of an animal is influenced by:
• Prolificness
– Two years selection in corn could produce
327,680 Billion descendants for planting from 2
kernels selected
– Swine have bigger litters than cattle
How extensively you can make use of an animal is
influenced by:
• Generation time
– Poultry completes a generation in 7 or 8 months
– Sows farrow first at 1 year of age, 2 litters/year
– Cow calves first at 2 or 3 years, say avg of 5 yrs
as a realistic practical average
• With long generation interval comes slower
rate of improvement
3. Observe/measure accurately the
traits carried by the animal
• Desirable traits
– Health
– Prolificness
• Regular heat periods, enough ova, conceive on 1st
service
• Males masculine and progressive, sufficient volume,
concentration of sperm
– Long life, longevity (in some animals)
• Cows more impt than pigs
Desirable traits, continued
• Efficient growth
• Quality of product
Factors that influence which traits to
emphasize
• Choose traits contributing most to long-term
profit
• Choose traits for which your herd is lacking
• Choose traits with a degree of heritability
Measure accurately
• Records
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Individuals, birth dates, litter size, birth weight
Use a SCALE, not eyeball
Proper ID of animals
Carcass measurements
• Backfat, loin eye area, carcass length, etc.
Animal Id Systems
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Tatoo
Ear notch
Ear tag
Leg band
Brand
Freeze brand
Paint
Age estimation
• Records are best
• Other methods
– Teeth
– Size of body parts
• Use of Central Testing facilities
– Pros
– Cons
• Systems for measuring meatiness
– Backfat probe (fat depth related to meatiness)
• Metal ruler
• Ultrasonics
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“Fat-o-meter”
TOBEC
Cutout info from relatives
Visual appraisal – OFTEN INACCURATE
Ways of assessing breeding efficiencies
of sires beforehand
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Sperm volume, concentration, viability
Libido
Scrotal circumference
In the female, it may be more difficult to
assess
– Mothering ability
– Temperament
Conformation
• Some of the “desired” terms are highly
unscientific
• Need adequate leg structure, mammary
system, etc.
• Some traits cannot be easily measured
How to consider multiple traits
• Realize selecting for many things means
less progress in each
• Selection Thresholds vs Indexes
• Indexes take into account relative economic
importance and heritability
Relatives
• Ancestors
– Animal gets just ½ its genes from a parent
– Is ¼ related to a grandparent
– Only 1/8 related to a great grandparent
• Sibs (brothers & sisters)
• Progeny
Progeny Testing
• The BEST info on what genes an animal
can pass into its offspring is what is seen in
its offspring
When to progeny test
• When you want the best answer badly
– Because it takes a long time to develop a parent
(long generation interval)
– Expensive procedure
– Use for Dairy cattle, Beef cattle
– Less used for poultry, swine
• Use of offspring selected on other info can make
more rapid progress even if less accurate
Repeatability
• Will sow with large litter 1 have another?
• Does production repeat in next season?
• Depends on the trait
Repeatability
Trait
Prolificacy
Birth wt
Repro Effic.
Weaning wt.
Grade at wean
Annual wool
Beef
.30
.10
.45
.22
Sheep
.10
.30
.20
.40
.50-.60
Swine
.10-.16
Use the selected animal most
effectively
• AI
• Embryo transfer
Hybrid Vigor = Heterosis
• Increased vigor of crossbreds as compared
with the average of the purebred parents
– High for “non-additive” traits
• Maternal ability
• Survival
– Low for : carcass, growth rate
Heterosis
• Can NOT be transmitted from one
generation to the next
• MUST be recreated in each generation by
making the cross
Heterosis
• How important? Everybody? Forever?
Mating Systems
• Random mating
– Selected males and females run together
• Example: bulls with cows in range country
• Inbreeding
– Mating relatives
– Used to concentrate genes (homozygosity)
• Concentrates good genes AND bad genes
• Outbreeding
– Similar to crossbreeding, but still within a
breed
– Mating animals that are “unrelated”
Crossbreeding
• Mating animals from different breeds
• Need a system to make progress
successfully
• Reasons for crossbreeding
– Bring in good genes lacking in a breed
– Increase vigor, prolificacy
Crossbreeding methods
• Rotational crosses
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2 way
3 way
How many breeds?
Advantage – can produce your own females
Terminal Cross
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Select a male line and a separate female line
Sell all resulting offspring
Purchase males, females
Advantage:
– Male line can be great at carcass & doesn’t
have to be great at maternal traits
– Female line can emphasize female traits
The future
• “Breeds” per se became less important in
poultry, likely swine will follow
• Identification of specific genes of benefit
• Marker assisted selection
• Transgenics and other biotechnology tools
• “Genomics”
• Maybe “the future is now”