Breeding and Genetics 101

Understanding
Breeds

Outline
• 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 (nongenetic) factor has on phenotype

G x E
Interaction
90
80
70
60
50
40
30
20
10
0
Temperate Tropical
Temperately
Adapted
Tropically
Adapted

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

DNA

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
Homozygous
Polled
H
H h
Hh
Polled
Hh
Polled h
Hh
Polled
Hh
Polled
Homozygous
Horned
100%
Polled

Punnett Square
Hetrozygous
Polled
H h
H h
HH
Horned
Hh
Polled
Hh
Polled hh
Polled
Heterozygous
Polled
75%
Polled

Punnett Square
Homozygous
Horned
H
H
H h
HH
Horned
HH
Horned
Hh
Polled
Hh
Polled
Heterozygous polled
50%
Polled

Gene
Interaction
• Epistasis – An interaction among different loci such that the expression of one gene depends on another

eD eD eD eD
Epistasis
Ed
EeDd
EeDd
EeDd
EeDd
Angus
Ed Ed
EeDd
EeDd
EeDd
EeDd
EeDd
EeDd
EeDd
EeDd
Ed
EeDd
EeDd
EeDd
EeDd
100% Black Coat Partial Diluted = Grey

eD ed
ED eD
Epistasis
ED
EEDD
EeDD
EeDD
EeDd eD
A x C
EeDD
EeDD eeDD eeDd
Ed
EEDd
EeDd
EeDd
Eedd ed
EeDd
EeDd eeDd eedd

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
Calving interval
Birth Weight
Weaning Weight
Mature Weight
Feed Conversion
Scrotal Circumference
Backfat Thickness
Marbling Score h 2
.05
.40
.30
.40
.40
.50
.40
.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)

Questions