Lecture 3: Basic Genetic Model for Polygenic

What does the Mozart Example emphasize?

Becoming a musical genius like Mozart requires:

• Genetic predisposition

• Environment (training, exposure, opportunity)

Emphasizes that genes matter, but environment shapes expression

True or False: A calf may have high genetic potential for growth, but poor nutrition prevents expression, so phenotype does not equal genotype

True

Characteristics of polygenic traits

• Controlled by many genes

• Each gene contributes small effects

• Environment strongly influences outcome

• Cannot determine genotype from phenotype

Examples of polygenic traits in livestock

• Milk yield

• Weaning weight

• Fertility

• Growth rate

What is the foundational equation for the basic genetic model?

P = μ + G + E

What does P represent in the basic genetic model equation?

P = Phenotypic value (what we measure)

What does µ represent in the basic genetic model equation?

µ = Population mean

What does G represent in the basic genetic model equation?

G = Genotypic value

What does E represent in the basic genetic model equation?

E = Environmental deviation

Explain what the basic genetic model equation (P = μ + G + E) represents in words

Every animal’s phenotype (P) is:

1. The average of the population (μ)

2. Plus its genetic superiority/inferiority (G)

3. Plus environmental effects (E)

Suppose:

Population mean (μ) = 500 lbs

Calf A:

• G = +40 lb

• E = -10 lb

Calf B:

• G = +10 lb

• E = +30 lb

What is the phenotypic value for each calf? Which calf weighs more? Which calf has better genetics?

Calf A:

• P = 500 + 40 + (-10) = 530 lb

Calf B:

• P = 500 + 10 + 30 = 540 lb

Calf B weighs more — but Calf A has better genetics

This is why we separate G from E

What are the two parts of Genotypic Value (G)?

G = BV + GCV

What does BV stand for?

Breeding Value (additive effects)

What does GCV stand for?

Gene Combination Value (non-additive effects)

What is Breeding Value (BV)?

BV = Sum of independent allele effects

What does Breeding Value (BV) represent?

It represents the part of genetics that is transmitted to offspring

True or False: Breeding Value (BV) is the most important concept in animal breeding

True

Why is Breeding Value (BV) the most important concept in animal breeding?

Because only additive effects pass predictably to progeny

Example: Additive Model

Suppose:

• Allele A adds +2 lb

• Allele a adds 0

What are the genotypes and allele effects?

Genotypes:

• AA —> 2 + 2 = +4

• Aa —> 2 + 0 = +2

• aa —> 0 + 0 = 0

This is called an additive genetic model because:

• Each allele contributes independently

• We just add their effects together

True or False: In an additive genetic model, there is no dominance and no interaction. Just addition

True

Breeding value is the total additive effect of all alleles the animal carries. If an animal has many loci affecting weight, we sum all additive effects across those loci.

So when we say:

• BV = +20 lb

That means what?

This animal’s genes make it genetically 20 lb above the population average (due to additive effects)

Why doesn’t the offspring of an animal with BV = +20 get all 20?

Because an animal has:

• 2 alleles per locus

• But passes only ONE allele per locus to each offspring

So, it only passes half its genes to each calf

Therefore, transmitting ability (TA) = BV/2

Simple One-Locus Example: Suppose a ram is AA (each A adds +2). What is the ram’s BV at this locus?

AA —> 2 + 2 = +4

What happens to the BV when the AA ram makes sperm?

Each sperm gets:

• Either A

• Or A

But only ONE A goes to each offspring

So each lamb only gets +2 from him at that locus, not +4

Suppose:

Population mean = 500 lb

Bull BV = +20

Cow BV = +10

What is the transmitting ability (TA) of the bull and cow? What is the expected calf average BV (Expected Progeny Difference (EPD))?

Bull TA = (BV / 2) = (20 / 2) = +10

Cow TA = (BV / 2) = (10 / 2) = +5

EPD = 500 + 10 + 5 = 515 lb

Animal A:

• BV = +40

• Environment = -20

• Phenotype = +20

Animal B:

• BV = +10

• Environment = +30

• Phenotype = +40

Which animal will look better? Which animal will produce genetically superior offspring? Why?

Animal B will look better because it has a higher phenotype than Animal A

Animal A will produce genetically superior offspring because breeding decisions depend on BV, not phenotype, and Animal A has a higher BV than Animal B.

What does Gene Combination Value (GCV) include?

• Dominance

• Epistasis

These depend on specific allele combinations

True or False: GCV is NOT predictably passed to offspring

True: If dominance makes Aa superior, you cannot guarantee offspring will be Aa

What is the expanded genetic model equation?

P = μ + BV + GCV + E

True or False: Each parent passes half its BV

True

What is the equation for transmitting ability (TA)?

TA = BV / 2

Using TA, what is the equation for expected progeny performance?

P(offspring) = μ + TA(sire) + TA(dam)

What does Expected Progeny Difference (EPD) stand for?

Different in transmitting ability

What animal is Expected Progeny Difference (EPD) used heavily in?

Beef cattle

Expected Progeny Difference (EPD) Example:

Bull A:

• EPD for weaning weight = +40 lb

Bull B:

• EPD for weaning weight = +20 lb

What is the difference in EPD between Bull A and Bull B? If bred to similar cows, what is expected of Bull A’s calves?

Difference = 20 lb

If bred to similar cows:

• Bull A’s calved are expected to average 20 lb heavier

What does µ mean?

Population mean

Is µ passed to offspring?

Not applicable

What does BV mean?

Additive effects

Is BV passed to offspring?

Yes

What does GCV mean?

Dominance/epistasis

Is GCV passed to offspring?

No (not predictably)

What does E mean?

Environment

Is E passed to offspring?

No