5.2: Heritability

Heritability estimates the relative contribution of genetic factors to the phenotypic variability observed in a population. What causes variance among organisms and among lines or varieties or strains or ecotypes? Remember, phenotypic variation observed among organisms is due to differences in

• their genetic makeup,
• environmental influences on each plant or genotype, and
• interaction of the genotype and environment.

We concern ourselves with the heritability of a trait -- the proportion of variation explained by genetic factors -- because only genetic effects can be transmitted to progeny. Heritability estimates

• the degree of similarity between parent and progeny for a particular trait, and
• the effectiveness with which selection can be expected to take advantage of genetic variability.

Family Resemblance

It's useful to think of the heritability of a trait through the lens of "family resemblance." Two relatives, such as a parent and its offspring, two full or half-siblings, or identical twins, would be expected to be phenotypically more similar to each other than either is to a random individual from a population. Although close relatives may share not only genes (they may also share similar environments for traits that have a large genetic component), resemblance between relatives is expected to increase as closer pairs of relatives are examined because they share more and more genes in common. In this conceptual framework, heritability can be understood as a measure of the extent to which genetic differences in individuals contribute to differences in observed traits.

Figure 11 Individuals that are related genetically would be expected to be more phenotypically similar to each other than to other individuals from a population.

Statistical Basis for Understanding Heritability

From a quantitative standpoint, heritability is defined as the proportion of the phenotypic variance that is explained by genetic variance. Variation in phenotypic variance \( \sigma^{2}_{p} \) is the sum of variation due to genetic factors (genotypic variance, \( \sigma^{2}_{g} \) ) and variation due to environmental factors (environmental variance,  \( \sigma^{2}_{e} \) ). Thus,

\[ \sigma^{2}_{p} =  \sigma^{2}_{g} + \sigma^{2}_{e} \]

and

\[ \text{Heritability} = \frac{\sigma^{2}_g}{\sigma^{2}_{p}} \]

Additionally, there are three seperate genetic phenomenon that contribute to genotypic variance:

• The additive effects that result from the contributions of different alleles result in additive variance,  \( \sigma^{2}_{A} \)
• The dominance effects that result from some alleles being dominant over others result in dominance variance,  \( \sigma^{2}_{D} \)
• The epistatic effects that result from interactions among alleles at different loci result in epistatic variance, \( \sigma^{2}_{E} \)

Genetic variance is the sum of the variance that arises from these three phenomena:

\[ \sigma^{2}_{g} = \sigma^{2}_{A} + \sigma^{2}_{D} + \sigma^{2}_{E} \]

 

Types of Heritability

There are two types of heritability: broad-sense and narrow-sense heritability.

Broad-Sense Heritability

Broad sense heritability, H², estimates heritability on the basis of all genetic effects. It is the proportion of the total phenotypic variance \( \sigma^{2}_{p} \) that is due to genetic variance \( \sigma^{2}_{g} \):

\[H^{2} = \frac{\sigma^{2}_g}{\sigma^{2}_{p}} \]

Narrow-Sense Heritability

In contrast, narrow-sense heritability, h², expresses the percentage of genetic variance that is caused by additive gene action \( \sigma^{2}_{A} \)

\[h^{2} = \frac{\sigma^{2}_{A}}{\sigma^{2}_{p}} \]

Narrow-sense heritability is always less than or equal to broad-sense heritability because narrow-sense heritability includes only additive effects, whereas broad-sense heritability is based on all genetic effects.

The usefulness of broad- vs. narrow-sense heritability depends on the generation and reproductive system of the particular population. In general, narrow-sense heritability is more useful for describing similarity of relatives than broad-sense heritability since only additive gene action can normally be transmitted to progeny. This is, because in systems with sexual reproduction, only gametes (alleles) but not genotypes are transmitted to offspring. In contrast, in case of asexual reproduction, genotypes are transmitted to offspring.

Table 6 Comparison of broad- and narrow-sense heritability.

Broad-sense heritability	Narrow-sense heritability
Symbols used	H^2, H, hb^2, or hB^2	h^2, hn^2, or hN^2
Predictor of Gain	Poor	Better
Genetic Variance	Additive, dominance, and epistatic	Additive only
Generation	Early	Later
Reproductive System	Self-pollinated or cloned population	Cross-pollinated

Estimating Broad-Sense Heritability - Breeding Inbred Strains

Inbred strains (of plants, of mice, etc) are particularly useful for estimating broad-sense heritability of a trait because we can assume that each individual is genetically identical -- genetic variance is 0. Thus, any variance we measure in a trait in an inbred strain is due solely to environmental factors. Similarly, when we cross two inbred strains, the hybrid F1 individuals remain genetically identical because we assume they are now they are heterozygous at each locus (one allele from each parental strain.)

What happens when we allow F1 individuals to cross with eachother? Now, the variance in the trait we're studying increases because there is now genetic diversity in the population! And now we can estimate H² because we are measuring variance in the trait due to both environmental and genetic factors. We've got \( \sigma^2_{e} \) and \( \sigma^2_{a} \) and we can use them to find \( \sigma^2_{g} \) and calculate H².

 

References

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Clausen, J., D. D. Keck, and W. M. Hiesey. 1948. Experimental studies on the nature of species. III. environmental responses of climactic races of Achillea. Carnegie Inst. Wash. Publi. 581.

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Department of Crop Science, University of Illinois at Urbana-Champaign. Values obtained for protein in the strains selected for oil and the values for oil obtained for the strains selected for protein each generation (1896-2004). 2007.