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 (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

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

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)

Questions