genetics. phenotype/genotype phenotype is what an animal looks like phenotype = genetics +...
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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
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
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 determines sex
• Other traits on that chromosome will be “sex linked” traits
• Mammals– Female = XX
– Male = XY
• (different nomenclature for poultry)
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
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
• Horse 64• Donkey 62• Mule 63• Swine38• Sheep 54• Cattle 60• Man 46
• Mink 30• Dog 78• Lion 38• Domestic cat 38• Bengal tiger 38• Chicken 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.
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
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
Cattle Swine
• No. of young weaned 10-15 10-15
• % lean cuts 40-50 30-40
• Rate of gain 50-55 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 2,000½ the genetics comes from bull, ½ from cowSo 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?
• 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
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– 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.
• Systems for measuring meatiness– Backfat probe (fat depth related to meatiness)
• Metal ruler
• Ultrasonics
– “Fat-o-meter”– TOBEC– Cutout info from relatives– Visual appraisal – OFTEN INACCURATE
Ways of assessing breeding efficiencies of sires beforehand
• 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
RepeatabilityTrait Beef Sheep
Swine
Prolificacy .10
Birth wt .30 .30
Repro Effic. .10 .20 .10-.16
Weaning wt. .45 .40
Grade at wean .22
Annual wool .50-.60
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
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– 2 way– 3 way– How many breeds?– Advantage – can produce your own females
Terminal Cross
• 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