ANIMAL SCIENCE 200 Introductory Animal Sciences

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• MEIOSIS
• production of gametes (oogenesis
  spermatogenesis)
• reduce number of chromosomes
• 1 parent diploid cell to 4 daughter haploid cells
  in sperm
• 1 parent diploid to 1 daughter haploid and 3
  polar bodies in oocytes

Figure 86  Meiosis.
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Allelic Interactions Dominance

• dominant gene overpowers and prevents
  expression of its recessive allele when
  heterozygous for a trait
• recessive gene a gene which is masked by its
  dominant allele when heterozygous for a trait

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Figure 87  The behavior of simple dominant and
recessive traits.
Allelic Interactions Dominance
What Genotypes would result from Rr X Rr?
25 RR 50 Rr 25 rr
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(Codominance)
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(Assumes each gene on a different chromosome)
Table not in text!
How has any uniformity of animals been achieved?
Degree of progress depends upon the nature of the
trait!!!
How many genes affect a given trait?
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Genetics types of traits

• Qualitative
• descriptive subjective measure
• classification into a few distinct groups
• examples hair color horns/polled blood types
• few genes control (often only 1 pair)
• environment has little if any influence
• progress not too difficult with 1 gene
• economic importance??

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Famous Detrimental Recessive Single Genes

• Can have devastating economic impacts!!
• Examples
• Dwarfism in Hereford Cattle
• Spider Lambs
• Mule Foot
• Often start with a popular sire with mutation

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Genetics types of traits

• Quantitative much greater economic importance!!
• objectively numerically measured
• continuous variation (unlimited of
  observations)
• many genes control (50, 100, or more pairs)
• examples milk production racing speed weight
  rib eye area

RACING SPEED
LOIN EYE AREA
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Quantitative Trait Example

• Growth Rate of an Animal what controls this?
• appetite, amount consumed, digestion,
  utilization, conversion
• best animal gets best allele for all genes that
  control these!
• influenced by environment?
• -are weight differences among animals due solely
  to genes?
• accuracy of measurement for qualitative vs
  quantitative?
• Gene mapping select on alleles, not on what
  they weigh!

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Genetics

• genotype genetic makeup of an animal
• phenotype characteristics of an animal that can
  be seen or measured (color, weight, speed)
• environment all non-genetic influences that
  affect animal traits (nutrition, season, age,
  health)

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Genetic change through selection

• Selection
• is creating different reproductive rates among
  animals (we want more offspring out of selected
  animals than others)
• increases frequency of desirable genes in the
  population (to increase growth rate, milk
  production, speed, etc.)
• Selection traditionally is performed on Phenotype
  

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Factors affecting rate of genetic improvement
when selecting for quantitative traits

• Heritability of the trait
• Selection differential
• Generation interval

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Heritability

• heritability for quantitative traits, the of
  phenotypic variation among animals for a
  particular trait that is due to heredity (not
  environment)

GREATER HERITABILITY GREATER IMPROVEMENT
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Selection Differential

• Selection differential
• superiority of selected animals compared to the
  herd average for a particular trait
• Example
• average of selected mares 62 inches
• average of all mares in herd 58 inches
• selection differential ??? inches

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Generation Interval

• average age of the parents when offspring are
  born
• (average age of breeding females average age
  of breeding males) / 2

swine 2 years horses 5-8 years dairy 3-4
years poultry 8-12 months beef 5-6
years sheep 3 years
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Genetic change per year

• heritability x selection differential

generation interval
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Mating Systems

• Designed to either
• Increase homozygosity - increase predictability
  of progeny as future breeding animals.
• Increase heterozygosity - increase performance of
  progeny produced for marketing or production.

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Mating Systems

• Inbreeding
• Intensive inbreeding - close relatives
• brother x sister, many generations
• widely used poultry industry
• e.g.inbred male/female lines for broilers
• increases homozygosity of gene pairs
• tests for undesirable recessive genes
• depresses performance
• increases predictability
• Linebreeding or Inbreeding in other species

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Animal Mating Systems

• 2. Crossbreeding
• mating animals of different breeds or lines
• beef, sheep, swine, dairy?
• Advantages
• breed complementation utilize different breeds
  with strength in traits of interest

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Animal Mating Systems

• 2. Crossbreeding (cont.)
• Advantages
• heterosis aka. hybrid vigor
• increased productivity of crossbred progeny
  above average of breeds or lines that are crossed
• greatest for lowly heritable traits

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Mating Systems

• Why does increased heterozygosity increase
  performance?
• Fewer unfavorable recessive genes expressed
• Favorable dominant genes are combined
• Heterosis or Hybrid Vigor same thing

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Tools for Effective Animal Selection

• Expected Progeny Difference (EPD) Beef Cattle
• expected performance of future offspring of an
  animal compared to the average of a group
• calculated from performance of relatives
  ancestors, siblings, progeny
• Traits
• GROWTH birth, weaning weight
• CARCASS rib eye area, fat thickness, marbling
• MILK PRODUCTION

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Tools for Effective Animal Selection

• Dairy Predicted Transmitting Ability (PTA)
• analogous to an EPD
• Milk, Protein, Fat, Cheese
• Somatic Cell Score, Productive Life
• Udder Structure, Teat Structure
• Swine
• Number born alive, 21 day litter weight
• backfat depth, days to 230 pounds
• Selection indexes for maternal or growth

SELECT SIRES, INC.
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Genetic Improvement
selection plus crossbreeding
selection, no crossbreeding
Level of Performance
crossbreeding, no selection
no selection, no crossbreeding
TIME