lec 9 - heritability

Heritability I

Definition:

The extent to which genes control the expression of a trait. Heritability plays a crucial role in understanding how traits are passed from parents to offspring through genetic transmission, offering insights into genetic contributions versus environmental influences.

Understanding Heritability (h²)

Heritability applies to:

  • A single trait measured on individuals in a specific population at a given point in time, making it a useful concept for scientists and breeders alike.

Usefulness:

  • Making genetic evaluations: Heritability aids in quantifying the genetic potential of breeding stock, enabling better decisions for the enhancement of desired traits.

  • Predicting response to selection: Knowledge of heritability allows breeders to forecast how much a specific trait will improve in future generations if they select for it.

  • Assisting breeders: It helps breeders determine if it is more efficient to improve traits through management practices (like nutrition) or genetic selection (breeding).

Key Concept:

Heritability describes inheritance in populations, not individuals, emphasizing its role in explaining the variability of trait expression attributed to genetic factors. It delineates the influence of genetics in contrast to environmental determinants in shaping the phenotype of organisms.

Expression of Heritability (h²)

Scale:

  • Heritability is expressed on a scale of 0 to 1 (or 0 to 100%), indicating the proportion of phenotypic variance that is attributable to genetic variance. A value closer to 1 indicates that genetics play a dominant role in the trait's expression, while a value closer to 0 suggests a greater impact from environmental factors.

Examples of Heritability Estimates:

  • Wool mean fibre diameter: Highly heritable (~75%), indicating a strong genetic influence on this trait.

  • Dairy milk concentration of fat and protein: Highly heritable (~70%), reinforcing the role of genetics in this critical production trait.

  • Growth rate: Moderately heritable (~60%), suggesting a balanced influence of genetics and environment.

  • Reproductive performance: Low heritability (1-2%), indicating that environmental factors may drastically influence reproductive success.

Take Home Messages

Genetic Diversity:

  • The genetic architecture of simple traits may involve complex multigenic and multiallelic systems, meaning that multiple genes with different alleles can influence a single trait.

  • The number of possible genotypes is calculated as n(n+1)/2, where n represents the number of alleles available for a particular trait.

  • Genetic expression can vary for several reasons, including epistasis, environmental effects, and gene-environment interactions. Breeding for seemingly simple traits might necessitate consideration of multiple alleles across various genes.

Application in Breeding Programs

Role of Heritability:

  • Heritability informs breeders about the level of confidence they can place in an individual’s observed phenotypic performance when selecting parents for the next generation.

  • Genetic gain in breeding is directly correlated with the heritability of the trait under selection. High heritability traits tend to yield significant improvements when selected for, while low heritability traits pose challenges.

High Heritability Traits:

  • Traits with high heritability (above 0.40 or 40%) suggest a strong correlation between phenotype and genetic merit, making them favorable candidates for selection in breeding programs.

Low Heritability Traits:

  • Traits with heritability below 0.15 or 15% demonstrate less genetic predictability; their economic significance may justify efforts to select for improvement despite the challenges. For example, mastitis resistance in livestock has a low heritability (0.12), indicating that management improvements may be more effective than genetic selection.

Heritability Estimates in Various Domains

  • Human Traits:

    • Height: 0.80, showcasing a high genetic influence on this trait.

    • Body mass index: 0.45-0.80, indicating variability influenced by both genetic and environmental factors.

    • General intelligence (adults): 0.40-0.60, suggesting a moderate to strong heritable component in cognitive capabilities.

  • Cattle:

    • Production traits: Generally highly heritable, facilitating targeted breeding.

    • Reproductive traits: Often exhibit low heritability, posing challenges for genetic selection efforts.

Genomic Evaluation Example

  • This method focuses on enhancing key traits like milk production in female dairy cattle, utilizing DNA parentage verification for more accurate breeding decisions.

Calculation of Heritability (h²)

Three Methods:

  1. Broad-sense Heritability:

    • The ratio of genotypic variance (VG) to phenotypic variance (VP).

    • Formula: h² = VG/VP

  2. Mean Trait Values:

    • Narrow-sense heritability calculates the proportion of selection in the parent generation reflected in the offspring.

    • Formula: h² = R/S

    • R = trait mean of progeny - trait mean of parent population before selection

    • S = trait mean of parental population after selection - trait mean of parent population before selection.

  3. Parent/Offspring Regression:

    • A statistical method to assess the relationship between parent and offspring traits, often represented as a simple slope of the best fit line.

Example of Mean Trait Values Method Calculation

  • Using weaning weight in livestock as an example:

    • Progeny mean wean weight: 35 kg

    • Parent mean wean weight before selection: 32 kg

    • Parent mean weight after selection: 36 kg

    • Calculating R and S yields an h² value of 0.75, indicative of significant heritability in weaning weight, emphasizing the potential for genetic improvement in this trait.

Heritability Notes (easy to learn ver.)

Definition:

Heritability is the extent to which genes control the expression of a trait. It helps understand how traits are passed from parents to offspring through genetic transmission, shedding light on genetic contributions versus environmental influences.

Understanding Heritability (h²)

  • Applies to: A single trait in a specific population at a given time.

  • Usefulness:

    • Genetic Evaluations: Helps quantify the genetic potential of breeding stock.

    • Predicting Selection Response: Forecasts how much a specific trait will improve in future generations.

    • Assisting Breeders: Determines if traits should be improved through management practices or genetic selection.

Key Concept:

Heritability describes inheritance in populations rather than individuals. It explains the variability of trait expression attributed to genetic factors versus environmental influences.

Expression of Heritability (h²)

  • Scale: Ranges from 0 to 1 (0-100%). A value closer to 1 indicates strong genetic influence; closer to 0 suggests greater environmental impact.

Examples of Heritability Estimates:

  • Wool Mean Fibre Diameter: Highly heritable (~75%).

  • Dairy Milk Fat and Protein Concentration: Highly heritable (~70%).

  • Growth Rate: Moderately heritable (~60%).

  • Reproductive Performance: Low heritability (1-2%).

Take Home Messages

  • Genetic Diversity: Complex multigenic and multiallelic systems may influence traits.

  • Genotypes Calculation: Number of possible genotypes is n(n+1)/2, where n is the number of alleles for a trait.

  • Variation in Genetic Expression: Influenced by epistasis, environmental effects, and gene-environment interactions.

Application in Breeding Programs

  • Role of Heritability: Indicates confidence in an individual’s observed performance for selection. High heritability traits yield better genetic gains.

  • High Heritability Traits: >0.40 (40%) are favorable for selection.

  • Low Heritability Traits: <0.15 (15%) show less predictability but can still be significant (e.g., mastitis resistance - 0.12).

Heritability Estimates in Various Domains

  • Human Traits:

    • Height: 0.80

    • Body Mass Index: 0.45-0.80

    • General Intelligence: 0.40-0.60

  • Cattle:

    • Production traits: Generally high heritability.

    • Reproductive traits: Often low heritability.

Genomic Evaluation Example

Enhances key traits like milk production in dairy cattle using DNA parentage verification.

Calculation of Heritability (h²)

Three Methods:

  1. Broad-sense Heritability: Ratio of genotypic variance to phenotypic variance. Formula: h² = VG/VP

  2. Mean Trait Values: Calculates proportion of selection in parents reflected in offspring. Formula: h² = R/S

    • R = trait mean of progeny - trait mean of parent population before selection

    • S = trait mean of parental population after selection - trait mean of parent population before selection.

  3. Parent/Offspring Regression: Assesses the relationship between parent and offspring traits with a slope of the best fit line.

Example of Mean Trait Values Method Calculation

Using weaning weight:

  • Progeny mean: 35 kg

  • Parent mean before selection: 32 kg

  • Parent mean after selection: 36 kgCalculating R and S yields h² = 0.75, illustrating significant heritability and potential for genetic improvement in weaning weight.