Animal Genetics: Genetic Models, Selection, and Mating Systems

Comprehensive vocabulary flashcards covering the genetic model, breeding values, heritability, repeatability, the key equation, correlated response, inbreeding, and crossbreeding systems from Animal Genetics (ANIM SCI 330).

Phenotypic Value ($P$)

A measure of an individual's performance for a specific trait, represented mathematically in the genetic model as $P = \mu + G + E$.

Population Mean ($\mu$)

The average phenotypic performance value of all individuals within a defined population, against which an individual's phenotypic performance is evaluated as a deviation.

Genotypic Value ($G$)

The overall effect of an individual's genetic makeup on its performance for a specific trait; it is not directly measurable and is partitioned into $G = BV + GCV$.

Breeding Value ($BV$)

An estimate of an animal's genetic merit for a trait, representing the additive and predictable component of the genotypic value transmitted from parent to offspring.

Gene Combination Value ($GCV$)

The non-additive and unpredictable component of the genotypic value caused by specific allelic interactions at a locus (dominance) or between loci (epistasis) that cannot be passed intact to progeny.

Permanent Environmental Effect ($E_P$)

A non-genetic factor with long-term consequences that permanently influences an animal's performance potential across all subsequent records (such as severe mastitis in dairy cattle).

Temporary Environmental Effect ($E_T$)

A non-genetic factor that impacts a single performance record of an animal but does not permanently alter its future performance potential (such as muddy track conditions on a race day).

Estimated Progeny Difference ($EPD$)

An estimate equal to half of an individual's breeding value ($\frac{1}{2}BV$ or $\frac{1}{2}EBV$) indicating how that individual's offspring are expected to perform compared to the population average; called Predicted Transmitting Ability ($PTA$) in dairy cattle.

Repeated Trait

A phenotypic trait that can be measured more than once over the lifespan of an individual, such as annual fleece weight, lactation yield, or litter size.

Most Probable Producing Ability ($MPPA$ / $PA$)

A prediction of an individual's future performance potential for a repeated trait that accounts for the population mean, genotypic value, and permanent environmental effects: $PA = \mu + (BV + GCV) + E_P$.

Parameter vs. Statistic

A parameter is a true numerical value summarizing an entire population (often difficult or impossible to measure), whereas a statistic is an estimate of a parameter calculated from a sampled subset of that population.

Heritability in the Broad Sense ($H^2$)

The proportion of total phenotypic variance attributable to total genetic variance: $H^2 = \frac{\sigma_G^2}{\sigma_P^2} = \beta_{G \cdot P} = r^2_{P,G}$; it measures the overall strength of the relationship between phenotype and genotype.

Heritability in the Narrow Sense ($h^2$)

The proportion of total phenotypic variance due solely to additive genetic variance (breeding values): $h^2 = \frac{\sigma_{BV}^2}{\sigma_P^2}$; values range from low ($\le 0.2$) to high ($\ge 0.4$).

Repeatability ($r$)

The correlation between repeated performance records for a trait on the same individual ($r = \frac{\sigma_{PA}^2}{\sigma_P^2}$), reflecting the proportion of phenotypic variation caused by differences in producing ability.

Contemporary Group

A management group of animals of similar age, breed, and sex raised under uniform environmental conditions, used to account for non-genetic variation by evaluating animals as deviations from the group mean.

Key Equation for Genetic Change

The foundational mathematical equation determining the rate of genetic change (response to selection) per unit of time: $\Delta_{BV}/t = \frac{r_{BV, \hat{BV}} \cdot i \cdot \sigma_{BV}}{L}$.

Accuracy of Selection ($r_{BV, \hat{BV}}$; $r_{BV, \hat{BV}}$)$$

The correlation between true breeding values and their predicted estimates ($\hat{BV}$); under direct phenotypic selection, it equals the square root of narrow-sense heritability ($\sqrt{h^2} = h$).

Selection Intensity ($i$)

A standardized measure of how elite chosen parents must be relative to the entire candidate population, calculated as $i = \frac{\bar{SC}_s - \bar{SC}}{\sigma_{SC}}$; it has an inverse relationship with the proportion of candidates saved.

Phenotypic Selection Differential ($S$)

The numerical difference between the average phenotypic performance of selected breeding parents ($\bar{P}_s$) and the average of all potential candidate parents ($\bar{P}$), represented as $S = i\sigma_P = \bar{P}_s - \bar{P}$.

Generation Interval ($L$)

The amount of time required to replace one generation with the next, mathematically defined as the average age of parents when their offspring are born.

Two-Path Method

A modification of the key equation that separates male and female parameters to account for the higher selection intensity and accuracy typically achieved in sires compared to dams.

Correlated Response to Selection

The genetic or phenotypic change observed in an unselected trait ($Y$) resulting as a secondary consequence of direct artificial selection applied to another trait ($X$).

Pleiotropy

A biological phenomenon where a single gene or locus directly influences two or more distinct phenotypic traits (e.g., the frizzle feather allele in chickens affecting feather structure, metabolism, and body temperature).

Halothane Gene (RYR1)

A mutation in the Ryanodine Receptor 1 gene in swine that increases leanness and feed efficiency, but causes reduced litter size, pale, soft, exudative (PSE) pork, and porcine stress syndrome (PSS).

Genetic Correlation ($r_{BV_X, BV_Y}$)

A statistical measure of the direction and strength of the relationship between breeding values for two different traits in a population, ranging from $-1.0$ to $+1.0$.

Correlation Sign and Strength

The classification of relationships between traits by direction (positive or negative), consistency (strong or weak), and economic or biological favorability.

Indicator Trait

A secondary trait ($X$) that is genetically correlated with an economically important target trait ($Y$), selected to indirectly improve trait $Y$ when the target trait is difficult, expensive, or sex-limited to measure directly.

Ratio of Response

The ratio of indirect genetic change in trait $Y$ (achieved by selecting for correlated trait $X$) relative to direct genetic change in trait $Y$; indirect selection is superior when this ratio exceeds $1.0$.

Inbreeding

The mating of animals that are more closely related to one another than the average relationship among individuals in the population, which systematically increases genomic homozygosity.

Prepotency

The capacity of an individual to produce offspring that strongly resemble that parent or show exceptional phenotypic uniformity, stemming from high levels of homozygosity.

Inbreeding Depression

The reduction in performance, vigor, or survivability of polygenic traits caused by increased homozygosity exposing unfavorable recessive alleles (manifestation of a poor gene combination value).

Inbreeding Coefficient ($F_X$)

The mathematical probability (ranging from $0$ to $1$) that both alleles at an arbitrary locus within an individual are identical by descent from a common ancestor.

Identity by Descent (IBD)

The state where two alleles are exact functional copies of an allele present in a shared ancestor, passed directly down pedigree branches to both lineages.

Identity by State (IBS)

The state where two alleles are chemically or phenotypically identical purely by chance or independent origin rather than through direct inheritance from a shared common ancestor.

Linebreeding

A controlled, mild form of inbreeding structured to maintain a high degree of genetic relationship to a single admired ancestor while keeping the overall inbreeding coefficient low.

Wright's Coefficient of Relationship

A measure of pedigree relatedness that calculates the expected correlation between the breeding values of two individuals due to shared common ancestry.

Fitness

The biological capability of an organism to survive and successfully produce fertile offspring in its environment; it is commonly depressed by inbreeding and enhanced by outbreeding.

Outbreeding

The mating of individuals that are less closely related than the average relationship of the population, which increases heterozygosity, masks deleterious recessive alleles, and produces hybrid vigor.

Hybrid Vigor (Heterosis)

The phenotypic superiority in performance of crossbred progeny ($F_1$) over the average performance of their purebred parental breeds: $HV = \bar{P}_{F_1} - \bar{P}_P$, driven by favorable gene combination values.

Overdominance

A form of gene action where the heterozygous genotype displays a phenotypic expression that exceeds both homozygous dominant and homozygous recessive parental types, contributing to heterosis in polygenic traits.

Composite Breed

A synthetic breed developed by crossing two or more distinct breeds, designed to permanently capture heterosis and breed complementarity while mating strictly inter se (composites mated to composites).

Polypay Sheep

A four-breed composite sheep breed created from Finnsheep (prolificacy), Rambouillet (fleece), Targhee (fleece and body size), and Dorset (maternal ability).

Breed Complementarity

The production of superior crossbred offspring by mating breeds whose diverse, desirable genetic strengths compensate for each other's individual weaknesses (e.g., crossing maternal dams with terminal sires).

Terminal Crossbreeding System

A commercial crossbreeding strategy where all crossbred male and female progeny are sold as terminal end-products (e.g., meat or slaughter) and no female replacements are saved within the herd.

Rotational Crossbreeding System

A crossbreeding system where replacement females are retained and sequentially mated across generations to sires belonging to the breed most genetically different from their own.

Beef on Dairy

A specialized breeding program where top-tier dairy cows receive sexed semen to generate dairy replacements, while lower-merit cows are bred to beef sires to yield high-value crossbred feedlot calves.