Genetics Notes

Basic Genetic Terminology

• Gene- active area on the chromosome that codes for a trait and determines how that trait will develop
• Alleles- alternative for of a gene
• Homozygous- like alleles “BB or bb”
• Heterozygous- different alleles “Bb”

Basic Genetic Terminology

• Genotype- genetic makeup
• “BB” vs. “Bb” vs. “bb”
• Phenotype- the expressed characteristic
• Ex: color, horned vs. polled, birth weights

Qualitative Traits

• Primarily determined by genotype
• Little or no environment influence
• Determined/ controlled by small # of genes
• Often detected by visual observation
• Ex: hair color, horns

Single Gene Effects�(Genotype vs Phenotype)

• Basic Genetics
• Ex: Hair color
• B= Black (dominant)
• b= Red (recessive)

• Offspring inherit 1 allele from each parent for each gene

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2 phenotypes

3 genotypes

BB bb

Bb

Bb bb

½ Bb

½ bb

Punnet Square:

B

B

b

b

BB

Bb

Bb

bb

Quantitative Traits

• Most traits of economic importance
• Show continuous variation
• Cannot be separated into distinct groups
• Affected by many genes
• Impossible to determine effect of one gene pair
• Environmental affects gene expression

Quantitative Traits

• Can measure phenotype, but cannot determine the specific genotype
• Examples:
• Weights
• Milk production
• Fertility
• performance

Environmental Effects

• Known: effects that have an average effect on all individuals within a specific category
• i.e.: age of dam, gender
• Ex: weaning weight:
• Calves born earlier in the calving season are older at weaning and will be heavier at weaning
• Unknown: random in nature

Environmental Adjustments

• Adjust records to remove environmental effects
• Ex: weaning weight of calves
• Adj for age of calf, sex of calf, and age of dam
• Adj to a common age of 205 days

[( ) x 205] + BW +Age of dam adj.

Determine age of dam adj.

Male calf: 2 (+60), 3(+40), 4(+30), 5-10 (0), >10 (+20)

Female calf: 2 (+54), 3(+36), 4 (+18), 5-10 (0), >10 (+18)

……Actual WW-BW…..

Age at weaning, days

Quantitative Traits will have a Normal Distribution

• Bell- shaped curve

Normal Distribution

• Mean= the average
• Standard Deviation (SD)
• + or – 1 SD = 68% of the population
• + or – 2 SD = 95% of the population
• + or – 3 SD = 99% of the population

Continuous Variation

• Weaning Weight
• Avg: 600
• SD: 500

500

700

Mean: 600

+ or – 250 95% of population

You want this one!

740 #

Selection

• Important tool for genetic improvement
• Differential Reproduction
• Prevent some from reproducing
• Use others to become parents of many offspring
• Change gene frequency in the population
• Increase frequency of desirable gene (alleles)
• Decrease frequency of undesirable genes

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Selection

• Selection is based on phenotypes
• P= G + E
• Thus, proper selection of phenotype will change genotype of population slowly over time.

Continuous Variation

• Weaning Weight
• Avg: 600
• SD: 500

500

700

Mean: 600

140 # difference

You want this one!

740 #

Heritability

• % of the phenotypic variation that is genetic and transmitted from parents to progeny.
• Ranges from 0 – 100%
• It is an estimate

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Heritability of Production Traits

Traits Heritability (h²)

Reproduction Low (0 – 20%)

Growth Performance Medium

Mature Size

Carcass Traits High (>40%)

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Level of h² is an important determining factor for the accuracy of selection decisions

Expected Progress from Selection

• Factors affecting progress
• Heritability
• Selection Difference
• Difference between the trait value of the selected animal and the average value for the group from which they were selected.

Breeding Value

• The animals genetic superiority over the herd or population average
• Can be estimated by…
• Selection difference • h²

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140# x .3 = 42# BV

½ BV = 21# EPD

Expected Progeny Difference

• Only ½ of parent’s genes are transmitted
• = ½ of the breeding value
• The amount by which the average progeny of the animal is expected to exceed the average of the herd or population

EPD

• Our best tool for genetic selection
• Used to compare genetic potential
• Available for cattle, sheep, and swine
• Theoretical average = 0

Usage of EPD’s

• Weaning weight EPD
• Sire A = +20# of calf
• Sire B = +32# of calf
• Diff. = 12
• Calves of Sire B should weigh 12 lbs heavier (on average) than those of Sire A when bred to the same cows.

Usage of EPD’s

• Days to 250 lbs. EPD (swine)
• Boar A = -3.5 days
• Boar B = 0 days
• Diff. = -3.5 days
• Progeny from Boar A should reach 250 lbs. 3.5 days slower than progeny of Boar B if bred to sows of similar genetic merit.

Genetic Change Per Year =

• Factors that affect genetic change per year
• Heritability
• Selection difference
• Generation Interval

Generation Interval

• Average age of parents
• When the offspring are born
• Average Generation Values
• Swine= 2 years
• Beef Cattle= 5-6 years
• Dairy Cattle= 6-8 years
• Although cows are replaced frequently (25 – 30% cull rate)
• Tend to use older, proven sires

Genetic Change & Selection Methods

• Multiple trait selection slows change
• But, usually more desirable
• Too much progress in one trait may lower productivity in other traits
• ie: yearly weight and birth weight in cattle
• Traits may be positively correlated
• Some of same genes affect more than one trait
• Swine rate of gain and feed efficiency

Selection Method

• Tandem
• Independent culling levels
• Selection Index

Tandem Selection

• Single-trait selection
• Rapid change in a single, highly heritable trait
• Other traits may be negatively affected
• Usually not recommended

Independent Culling Levels

• Set minimum levels for each trait of importance
• Most common
• Work well with small number of important traits
• Only a small % of offspring kept as replacements
• Disadvantage:
• May cull a relatively superior animal for poor performance in a single trait.

Selection Index

• Recognizes value of multiple traits
• Economically weights value of different traits
• Provides overall ranking from best to worst
• Most effective
• Most difficult to develop
• Example indexes-
• Swine: SPI, TSI, MLI
• Cattle: $Feedlot, $Grid, $Beef, $Energy

Requirements of Effective Selection

• Heritable traits
• Relative easy of measurement
• Associated with economic value
• Accurate genetic estimates
• Available genetic variation

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Basis for using performance data

Does selection work?

• Turkey
• Since 1950’s have changed from thin-breasted wild turkeys to modern turkeys
• Wild turkeys: 10# in 6 months
• Modern turkeys: 27# in 5 months
• Combined genetic selection and increased environment

Does selection work?

Mating System

• Straight-breeding (pure bred)
• Mating animals of the same breed
• Crossbreeding
• Mating animals of different breeds

What is a breed?

• Animals of common origin
• Possess characteristics that distinguish them from other groups within the species
• Through selection & breeding, come to resemble & pass traits uniformly to offspring

Purebred vs. Fullblood

• Purebred animal is eligible for registration with a recognized breed association
• Fullblood animal has a pure ancestry, no other breed appears in the pedigree

Composites & Hybrids

• Composites
• Attempted to combine desirable traits of various breeds.
• Term used in cattle, sheep, and poultry.
• Hybrids– used in the swine industry
• Developed from specific line crosses
• Selected for specific traits
• Large corporate seedstock suppliers of boars & gilts
• ex: PIC
• Common (easy terms) definition
• Hybrid or composite carefully orchestrated crossbred animals.