Download 2. Breeding and Genetics 101

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
Performance
GxE
Interaction
90
80
70
60
50
40
30
20
10
0
Temperately
Adapted
Tropically
Adapted
Temperate
Tropical
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
h
Hh
Hh
Polled
Polled
Hh
Hh
Polled
Polled
Homozygous
Horned
100%
Polled
Punnett Square
Hetrozygous
Polled
H
h
H
h
HH
Hh
Horned
Polled
Hh
hh
Polled
Polled
Heterozygous
Polled
75%
Polled
Punnett Square
Homozygous
Horned
H
H
H
h
HH
Hh
Horned
Polled
HH
Hh
Horned
Polled
Heterozygous
polled
50%
Polled
Gene
Interaction
• Epistasis – An interaction
among different loci such
that the expression of one
gene depends on another
Epistasis
Charolais
Angus
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
Epistasis
AxC
AxC
ED
eD
Ed
ed
ED
EEDD
EeDD
EEDd
EeDd
eD
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
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