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Hello everyone, I welcome you to another lecture from the Animal Breeding module,

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the topic of which is Genetic parameters – specially heretability.

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In to basic genetic parameters include coefficient of heritability,

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coefficient of repeatability and genetic correlation.

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In this lecture, we will deal with the coefficient of heritability.

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For selecting a suitable selection method and, as a result, for the entire breeding process,

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knowledge of how is heredity is involved in the manifestation of a quantitative trait is of fundamental importance.

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We are therefore interested in how far individual performance are heritable.

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It is expressed by heritability coefficient, which we understand as the proportion of genetic variability to total phenotypic variability.

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It is, therefore, represents the share of gene variability in phenotypic variability.

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Also, to what extent is the genetic similarity of related individuals accompanied by phenotypic similarity?

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Genetic similarity is determined by relationships. The heritability coefficient is denoted h^2

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and indicates the proportion of genetic variability to phenotypic variability.

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As already said, the heritability coefficient expresses the proportion of total genetic variability

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to total phenotypic variability. We refer to this coefficient of heritability in the broad sense

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because it includes the entire genetically determined component of variability,

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that is, the component of additivity, component of dominance and component of interaction.

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However, we can estimate only the additive proportion of genetic variability based on mathematical statistical procedures.

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In this case, coeficient of heritability is equal to the following formula. And that's why we talk about heredity in a narrower sense.

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The heritability coefficient takes on values from 0 to 1.

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If we consider the first extreme, i.e. the heritability coefficient equal to 0,

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and if we told the total phenotypic variance equal to one, the genetic variability will be equal to zero,

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which means that the heritability coefficient will also be equal to zero, as follows from this formula.

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If we consider the second extreme, i.e. the heritability coefficient equal to 1,

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we would not encounter any differential action of environmental effects,

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and all phenotypic variance would be conditioned only by genetic differences so that the heritability coefficient

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would be equal to one according to this formula.

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If we only consider quantitative traits, both extremes are practically unattainable in the regular breeding of livestock.

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We could get a extreme value of 0 only in the case of studying clones where they have the same genetic background.

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The second extreme, when the coefficient of heredity is equal to 1, exist in the case of laboratory conditions,

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when we completely exclude different influences of environmental effects,

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or when it comes to qualitative traits that are not affected by the environment,

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i.e. traits is affected by genes of large effects and is governed by Mendelian laws.

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In other cases, the quantitative traits are characterized by the heritability coefficient values,

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which range between the two mentioned extremes.

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Quantitative traits we can divided into three groups according to the level of the heritability coefficients value.

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The first group includes traits with low heritability, its means that heritability coefficient value is lower than 0.3,

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where we include sub-characters of reproduction and fitness.

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In the second group, included traits with medium heritability, its means heritability coefficient values ranged from 0.3 (including) to 0.6,

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in this group we include sub-traits of production traits. And finally, in the third group,

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highly heritable characters, a heritability coefficient of this traits is higher than 0.6 (including).

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In this group we include sub-traits of slaughter value.

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As for the methods of determining the heritability coefficient,

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they are based on the assumption that the mutual similarity of the observed quantitative traits 

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is more similarly between related animals than between unrelated animals.

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Generally, it is possible to use the following approaches: From the similarity of parents and offspring,

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which includes procedures using regression or correlation coefficients and selection procedures (experiments).

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Another method is From the similarity of sibs, which provides the analysis of full sibs, half-sibs and combination of analysis of full sibs and half-sibs,

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and analysis of identical twins. Further: From the features' repeatability and the similarity of bilaterally occurring traits on the same individual.

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Here, we see an example of estimating the heritability coefficient using a realized effect of selection or genetic gain.

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In this method, we start from the relationship that the selection effect, or genetic gain, 

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is a function of the heritability coefficient and the selection difference.

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The heritability coefficient is a critical genetic parameter due to its importance in animal breeding.

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The value of the heritability coefficient predetermines the selection methods that can be used in breeding process.

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Roughly speaking, the trait with higher value of the heritability coefficient can be selected by the simpler selection methods, and conversely,

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the selection of trites with lower the value of the heritability coefficient, must be used more sophisticated selection method.

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For example, for trite with higher values of the heritability coefficient, can be possible selected based on own performance of individual. Conversely, 

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for traits with low value of coefficient heritabilty, can be used a selection based on performance of a large number of offsprings. 

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The value of the heritability coefficient is further used for the prediction of similarity of genotype and phenotype of individuals,

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similarity in the performance of parents - offspring, and estimate of the performance  of the offsprings,

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Estimate of individual heterosis and specific heterosis (in general, crossbreeding effects).

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This slide shows the average values of heritability coefficients for individual traits and species.

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As already mentioned, the heritability coefficient applies only to the populations on which it was estimated and is not transferable between populations.

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This is because each population shows different variability of phenotype and genotype.

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This lecture presented the concept of the heritability coefficient and its importance in animal breeding.

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Thank you for your attention, and I look forward to seeing you in the following presentations.