By :
Muhammad Tegar Kusmahidayat Konenda S.Pt.
First Expert Livestock Seed Supervisor
Livestock business practitioners make various efforts to increase livestock productivity. However, often the increase in productivity is only achieved through feed modifications and environmental control, neglecting genetic factors. Yet, genetic factors are one of the key determinants of livestock productivity. For example, previous studies have indicated that over 40% of dairy cow milk production is influenced by genetics. The question is, how can we increase livestock productivity while considering genetic factors? One way is through selection. Choosing animals with higher productivity from the population and further developing them. Now, one thing that can be said to be crucial in conducting selection is heritability.
What is Heritability?
In general, heritability can be divided into two meanings. First, broad-sense heritability, which is the comparison between the genetic variance, which is a combination of additive genetic variance, dominance, and epistasis, with the phenotypic variance.
Broad-sense heritability can only explain how much of the phenotypic variation is caused by genetic influence and how much by environmental factors. However, it cannot explain the proportion of phenotypic variation in parents that can be passed down to their offspring. It is known that the genotype of an animal is not entirely inherited by its offspring. Advantages of an animal caused by genes that act dominantly and epistatically will be divided during the process of crossing over and segregation in meiosis. Therefore, broad-sense heritability is not useful in animal breeding.
Second, narrow-sense heritability, which is the comparison between additive genetic variance and phenotypic variance. Narrow-sense heritability is also referred to as heritability and is denoted as h2.
Is Knowing Heritability Important?
Heritability is one of the most important considerations in animal evaluation, selection methods, and mating systems. Specifically, heritability is a part of the total variation in traits caused by genetic differences among observed animals. Heritability is the comparison between genetic variance and phenotypic variance. Phenotypic variance is influenced by genetic and environmental factors.
Heritability is a partial description of a trait in a group of animals under certain conditions. Variation may occur during the same time period between animal groups or variation within the same animal group at different times. Naturally, these differences may occur due to genetic and environmental differences between groups from year to year.
Theoretical values of heritability range from 0 to 1, but extreme values of zero or one are rarely found in quantitative animal traits. Traits with heritability value of zero are those where all phenotypic variation in animals is solely caused by environmental factors, and genetic influence is assumed to be absent. A heritability value of 1 indicates a quantitative trait where all variation in the trait is caused by genetic factors.
Heritability values are categorized into three categories: low, moderate, and high. Heritability is considered low (small) if its value is 0 - 0.2; moderate: 0.2 - 0.4, and high if it is greater than 0.4. Preston and Willis (1974) classify heritability values as low if less than 0.25, moderate if between 0.25 - 0.50, and high if greater than 0.50. According to Hardjosubroto (1994), heritability values are considered low if they are less than 0.10; moderate if between 0.10 - 0.30, and high if greater than 0.30.
How to Calculate Heritability
There are many ways to obtain heritability values. One method may not necessarily yield the exact same value as another method. The calculation of heritability is usually based on estimating variance components. Typically, this is done on newly formed populations.
Another method to estimate heritability is through regression. In breeding, this method is known as parent-offspring regression (PO regression). Heritability estimation is based on kinship relationships, namely half-siblings and full siblings. For outcrossing, if progeny from half-siblings is regressed with a single parent, then h2 = 2b, where b = Cov (P, O)/Var (P). For outcrossing involving full siblings, if the offspring is regressed with the mid-parent between F1 and F2, or F2 and F3, and so on, then h2 = b. With this parent-offspring regression, estimation may be biased if the assumptions used (no relationship between parents P1 and P2 or no additive gene effects, or different scales) do not apply. Therefore, further testing involves corrections due to these relationships.
For estimating broad-sense heritability in another way, it can be obtained by planting both F1 and F2 populations from the same individual in an experiment. The variation in F1 represents environmental variance, while the variation in F2 includes genetic and environmental variance. Thus, the heritability of that characteristic is:
h2bs = σG^2 / (σG^2 + σE^2) = (σF2^2 – σF1^2) / σF2^2
Another example is when we have a set of both parent populations (A and B), F1 from A x B, backcross F1 to each parent [BC1 = (A x B) B and BC2 = (A x B) A], and F2 from A x B crossing. Estimation of heritability based on this population will be better due to the careful estimation of environmental variance, based on the means of A, B, and A x B. These three populations are expected to not segregate and provide better estimates of environmental variance than F1 alone. By including both parent populations, F1, BC1, BC2, and F2, we can estimate narrow-sense heritability, in this case:
h2ns = 1/2 x σA^2 / (1/2 σA^2 + 1/4 σD^2 + σE^2), where the numerator can be obtained from: 2σF2^2 – (σBC1^2 + σBC2^2). The denominator is the variance of F2 itself.
REFERENCES
Gunadi, B. Robisalmi, A., Setyawan P., and Lamanto. 2015. Heritability and Selection Response of F-3 Population of Blue Nile Tilapia Seeds (Oreochromis Aureus) at Fry Stage. Research Institute for Fish Breeding.
Kurnianto, Edi. 2009. Animal Breeding. Graha Ilmu, Yogyakarta.
Noor, Ronny Rachman. 2010. Animal Genetics. Penebar Swadaya, Jakarta.
Rusfidra. A. 2006. The Importance of Heritability in Animal Breeding. https://bunghatta.ac.id/artikel-138