HERITABILITY: GENETIC BASES, STUDY METHODS, EXAMPLES - BIOLOGY - 2025

The heritability is the property that has a measurable phenotypic characteristic of a population to be shared or inherited through genotype. Generally, this trait or character is passed down from their parents to their descendants.

The phenotypic expression (which corresponds to the visible traits of an individual) of an inheritable character is susceptible to the environment in which the offspring develop, so it will not necessarily be expressed in the same way as in the parents.

Inheritance pattern of blood type between AB & O parents (Source: AB & O_RegularInheritance.PNG: user: Dr.saptarshiderivative work: Ksd5 via Wikimedia Commons)

In populations of experimental organisms it is relatively easy to determine what the heritable traits are, since the expression of a trait of a parent in the offspring can be observed by raising the offspring in the same environment as where the parents develop.

In wild populations, on the other hand, it is difficult to distinguish which are the phenotypic characters transmitted by heredity and which are the product of changing environmental conditions, that is, which are epigenetic changes.

This is especially difficult to distinguish for most phenotypic traits in human populations, where it has been suggested that the best models for study are identical twin pairs who are separated at birth and who grow up in the same environment.

One of the first scientists to study heritability was Gregor Mendel. In his experiments, Mendel obtained pea plant lines with characters that were inherited and expressed almost entirely between parents and offspring.

Genetic bases for the study of heritability

Heritability is the result of the transfer of genes through gametes (from parents to offspring) through sexual reproduction. However, during gamete synthesis and fusion, two recombinations occur that can alter the arrangement and sequence of these genes.

Scientists working on the experimental identification of heritable traits work with pure lines, isogenic for most of the loci (genetically identical), since individuals from pure lines have the same genotype in a homozygous way.

Isogenic lines guarantee that the architecture of the genes in the nucleus does not affect the phenotype that is observed, since, despite the fact that individuals share the same genotype, by varying the position of the genes in the nucleus, variations in the phenotype.

For researchers, obtaining pure and isogenic lines is a kind of "guarantee" that the phenotypic traits shared by parents and descendants are the product of the genotype and, therefore, are fully heritable.

Mendelian inheritance of fur color traits in cattle (Source: Sciencia58 via Wikimedia Commons)

Despite the fact that the phenotype is always the product of the genotype, it is important to bear in mind that, although individuals have the same genotype, it may happen that not all the genes are expressed in said phenotype.

Guaranteeing the expression of genes is a very complex study, since their expression may differ for each genotype and, on occasions, these genes are regulated by other factors such as epigenetic factors, those of the environment or other genes.

Study methods

The branch of genetics known as "Classical Genetics" focuses on the study of the heritability of traits. In classical genetics, parents are crossed with descendants of entire populations for several generations, until obtaining pure and isogenic lines.

H2 statistic

Once the heritability of a trait has been demonstrated, the degree of heritability can be quantified by a statistical index identified as H2.

Heritability (H2) is calculated as the ratio between the variances of the genotypic means (S2g) and the total phenotypic variance of the population (S2p). The phenotypic variance of the population can be decomposed into the variance of the genotypic means (S2g) and the residual variance (S2e).

The heritability statistic (H2) tells us what proportion of phenotypic variation in a population is due to genotypic variation. This index does not indicate what is the proportion of an individual phenotype that can be assigned to its inheritance and its environment.

It must be taken into consideration that the phenotype of an individual is a consequence of the interaction between its genes and the environmental conditions in which it develops.

Modern techniques

Currently, there are tools such as Next Generation Sequencing (SNG) with which it is possible to sequence the entire genome of individuals, so that inheritable traits can be tracked in vivo in the genome of organisms.

In addition, modern bioinformatics tools allow the nuclear architecture to be fairly accurately modeled to roughly locate genes within the nucleus.

Examples

- Heritability in plants

The statistical method to measure the degree of heritability of the characters was proposed for crop species with commercial interest. Therefore, most of the examples in the literature relate to plant species important to the food industry.

In all crop species the heritability of the characters of agronomic interest such as resistance to pathogens, fruit yield, resistance to hot or cold temperatures, foliage size, etc. is studied.

The classic genetic improvement of vegetable crops such as tomato, seeks to select plants with a genotype that have heritable characters to obtain tomatoes that are larger, red and resistant to humid environments.

In grass species such as wheat, the aim is to select the heritable characters for size, starch content, and seed hardness, among others. With this objective, the varieties from different places are mixed until obtaining pure lines of each one.

By obtaining the pure lines, these can be combined into a hybrid variety, through genetic engineering, to obtain transgenic crops that gather the best characters in a single variety.

- Heritability in humans

In medicine, it is studied how some personality disorders are transmitted between parents and descendants.

Chronic depression, for example, is a phenotypic trait that is a product of the genotype, but if people with that genotype live in a familiar, happy, stable, and predictable environment, the genotype may never be seen in the phenotype.

Behavioral genetics is of particular interest in determining the heritability of intelligence quotient (IQ). To date, high levels of IQ have been found to be as heritable traits as a normal IQ.

However, a high IQ or chronic depression are expressed depending on the stimulation of the environment.

A typical example of heritability is the character of stature. If the parent is tall, the offspring are most likely tall. However, it would be clearly wrong to believe that, in an individual's height, 1.80 m is due to genes and another 0.3 m is due to the environment.

In many cases, longevity has also been studied as a heritable trait. For longevity studies in humans, the genealogy of the family is carried out, trying to incorporate data from the environment in which each of the individuals in the genealogical tree lived.

Most longevity studies have found that this trait behaves as a heritable trait in most cases and even increases in each generation if it is raised in the right environment.

References

1. Bratko, D., Butković, A., & Vukasović Hlupić, T. (2017). Heritability of personality. Psihologijske fears, 26 (1), 1-24.
2. de los Campos, G., Sorensen, D., & Gianola, D. (2015). Genomic heritability: what is it? PLoS Genetics, 11 (5), e1005048.
3. Devlin, B., Daniels, M., & Roeder, K. (1997). The heritability of IQ. Nature, 388 (6641), 468.
4. Griffiths, AJ, Wessler, SR, Lewontin, RC, Gelbart, WM, Suzuki, DT, & Miller, JH (2005). An introduction to genetic analysis. Macmillan.
5. Mousseau, TA, & Roff, DA (1987). Natural selection and the heritability of fitness components. Heredity, 59 (2), 181.
6. Vukasović, T., & Bratko, D. (2015). Heritability of personality: a meta-analysis of behavior genetic studies. Psychological bulletin, 141 (4), 769.
7. Wray, N., & Visscher, P. (2008). Estimating trait heritability. Nature education, 1 (1), 29.