breeding and genetics 101. understanding breeds outline what's the “best” animal –traits,...
Post on 28-Dec-2015
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• What's the “Best” animal– Traits, Genotypes, Phenotypes
• Genes, Chromosomes and Genotypes– Dominance and Epistasis– Simply inherited vs. polygenic traits
• Statistics and Their Role in Animal Breeding.
– Variation, Heritability, Genetic Evaluation
• Hybrid Vigor/Heterosis• The Genetic Model
– Breeding Value, Producing Ability
What's the “Best”
• Animals are usually characterized in terms of appearance or performance or a combination of both.
• Traits – Any observable or measurable characteristic of an animal.
– Coat color– Muscling– Weaning Weight– Marbling– Temperment
Traits and Phenotype
• Traits do not describe an individual.– A red calf has weighted 576 lbs at weaning.
• Phenotype – an observed category or level of performance for a trait.– Red and 576 are phenotypes for coat color and weaning wt
What Determines Phenotype?
• P = G + E• Genotype – the genetic makeup
of an animal• Environmental Effects – the
effect of an external (non-genetic) factor has on phenotype
G x E Interaction
0102030405060708090
Temperate Tropical
Perform
ance
TemperatelyAdapted
TropicallyAdapted
Genetics to Genes
• DNA – deoxyribonucleic acid– Genetic code of 4 bases
• Chromosome – a long strand of DNA, present in cell nucleus.
– Cattle have 30 pairs– Humans have 23 pairs– Dogs have 39 pairs
• Gene – the basic unit of inheritance.– Consists of DNA at a specific region on a chromosome
• Allele – alternate forms of a gene
Hetrozygosity
• Homozygous – posessing two copies of the same allele for a particular locus
• Heterozygous – having two different alleles for a particular locus
Dominance
• Dominant Allele - when present will override expression of any other allele for that trait
• Recessive Allele – can only be expressed when both recessive copies of the allele are present for a particular trait
Punnett Square
h h
H HhPolled
HhPolled
H HhPolled
HhPolled
Homozygous Horned
Homozygous Polled
100% Polled
Punnett Square
H h
H HHHorned
HhPolled
h HhPolled
hhPolled
Heterozygous Polled
Hetrozygous Polled
75% Polled
Punnett Square
H h
H HHHorned
HhPolled
H HHHorned
HhPolled
Heterozygous polled
Homozygous Horned
50% Polled
Gene Interaction
• Epistasis – An interaction among different loci such that the expression of one gene depends on another
Epistasis
Ed Ed Ed Ed
eD EeDd EeDd EeDd EeDd
eD EeDd EeDd EeDd EeDd
eD EeDd EeDd EeDd EeDd
eD EeDd EeDd EeDd EeDd
100% Black Coat Partial Diluted = Grey
Angus
Epistasis
ED eD Ed ed
ED EEDD EeDD EEDd EeDd
eD EeDD EeDD EeDd EeDd
eD EeDD eeDD EeDd eeDd
ed EeDd eeDd Eedd eedd
A x C
Simple and Polygenic Traits
• Simply inherited trait – a trait that is affected by only one gene
• Polygentic trait – a trait affected by many genes, no single gene having an overriding influence
Statistics
• Mean – the average
• Variation – differences among individuals within a population– Variance is the measure of variation– Covariation is how two traits vary together
Variation and Covariation
• Variation - Good or Bad?– Bad – adds to inconsistency– Good – Allows for increase selection efficiency– Measured as variance
• Covariation – Good or Bad?– Good – allows for one trait to indicate another– Bad – negative trait may be coupled with a positive– Measured as covariance– Correlation is the square root of covariance
Breeding Value
• Part of the animals genotypic value that is due to transmittable gene effects– The value of a parents genes to its progeny’s
performance– Not all of the genotypic value is heritable– An EPD is ½ the expected breeding value (EBV)
Heritability
• Many ways to look at it:– The extent that the difference we observe in
animal performance are due to inheritance.– A measure of the strength of the relationship
between performance (phenotypic values) and breeding values for a trait.
– It is a measure of correlation between breeding value and phenotype
Heritability Estimates
Trait h2
Calving interval .05
Birth Weight .40
Weaning Weight .30
Mature Weight .40
Feed Conversion .40
Scrotal Circumference .50
Backfat Thickness .40
Marbling Score .35
Heritability and Selection
• Heritability is critical to selection– The objective is to choose the animals with the
best breeding values to become parents– To do this we need the best info available, in most
cases phenotype is used– So the strength of relationship between phenotype
and breeding values is very important
Genetic Model
• Remember P = G + E?• Lets expand that to:
P = BV + GCV + E• GCV = Gene Combination
value– Includes Epistatic effects and heterorsis
Heterosis
• An increase in performance of hybrids over that of purebreds. Also caller Hybrid Vigor
• Results in increased heterozygosity• Hereford 500 lb wwt average• Angus 500 lb wwt average• H x A 525 lb wwt average
Heterosis
• More noticeable improvement in reproductive traits– Low heritability traits
• Heterosis effects are not heritable, must be created every generation
Genetic Model
• P = BV + GCV + E• The only heritable portion of
this is BV• All national cattle evaluation
programs center on prediction of BV (EBV)
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