Heritability is the proportion of phenotypic variation A phenotype is any observable characteristic or trait of an organism: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior . Phenotypes result from the expression of an organism's genes as well as the influence of environmental factors and the interactions between the two in a population that is attributable to genetic variation Genetic variation, variation in alleles of genes, occurs both within and among populations. Genetic variation is important because it provides the “raw material” for natural selection among individuals. Phenotypic variation among individuals may be due to genetic Genetics , a discipline of biology, is the science of heredity and variation in living organisms. The fact that living things inherit traits from their parents has been used since prehistoric times to improve crop plants and animals through selective breeding. However, the modern science of genetics, which seeks to understand the process of and/or environmental factors In epidemiology, an environmental factor are those determinants of disease that are not transmitted genetically. In more basic terms, an environmental factor is a factor caused, aided, or triggered by the environment. Heritability analyses estimate the relative contributions of differences in genetic and non-genetic factors to the total phenotypic variance In probability theory and statistics, the variance is used as one of several descriptors of a distribution. It describes how far values lie from the mean. In particular, the variance is one of the moments of a distribution. In that context, it forms part of systematic approach to distinguishing between probability distributions. While other such in a population.
Pay close attention to the variation part of "phenotypic variation": if a trait has a heritability of 0.5, it means that the phenotypic variation is 50% due to genetic variation. It does not imply that the trait is 50% caused by genetics.
Heritability is also specific to a particular population in a particular environment.
Overview
An example of low heritability: a population with genotypes coding for only one hair colourRather than look at all the traits of an organism, heritability focuses on the differences between multiple organisms for a single trait. Because heritability is concerned with variance, it is necessarily a description of a certain population - not an individual.
A population of asians The term "Mongoloid" is a historical racial category used to describe people of East Asia and Southeast Asian origin. Its use originated from a variation of the word "Mongol", a people who were considered one of its main proto-populations would contain individuals with genetics that code only for black hair. In this case, heritability is of course 0, since there is no variance in hair colour to analyse. But suppose some individuals dyed their hair and increased the population's variance in hair colours. Although now there are some differences in hair colour, theoretically heritability would still be 0 (i.e. 0% of the variance is due to differences in genetics).
A crowd with variance in hair colour.Oppositely, imagine a population of mixed races where hair dye is forbidden. Now there is again variance in hair colour, but this time heritability is 1 (i.e. 100% of the variance is due to differences in genetics). Of course, that would be assuming that our population's hair colours truly have nothing to do with environmental factors (like amount of sunlight). In practice, heritability is not that simple; environment and genetics interact.
Definition
Figure 1. Relationship of phenotypic values to additive and dominance effects using a completely dominant locus.Consider a statistical model A statistical model is a set of mathematical equations which describe the behavior of an object of study in terms of random variables and their associated probability distributions. If the model has only one equation it is called a single-equation model, whereas if it has more than one equation, it is known as a multiple-equation model for describing some particular phenotype:[1]
- Phenotype (P) = Genotype (G) + Environment (E).
Considering variances (Var), this becomes:
- Var(P) = Var(G) + Var(E) + 2 Cov(G,E).
In planned experiments, we can often take Cov(G,E) = 0. Heritability is then defined as:
- .
The parameter H2 is the broad-sense heritability and reflects all possible genetic contributions to a population's phenotypic variance. Included are effects due to allelic variation (additive variance), dominance variation, epistatic Epistasis is the phenomenon where the effects of one gene are modified by one or several other genes, which are sometimes called modifier genes. The gene whose phenotype is expressed is said to be epistatic, while the phenotype altered or suppressed is said to be hypostatic. Epistasis can be contrasted with dominance, which is an interaction (multi-genic) interactions, and maternal This article concerns the legitimate scientific concept of genes that are expressed only when carried by the female parent. It is not to be confused with the generally discredited theory of maternal impression and paternal effects, where individuals are directly affected by their parents' phenotype (such as with milk production in mammals).
These additional terms can be included in genetic models. For example, the simplest genetic model involves a single locus with two alleles that affect some quantitative phenotype, as shown by + in Figure 1. We can calculate the linear regression In statistics, linear regression is any approach to modeling the relationship between a scalar variable y and one or more variables denoted X. In linear regression, models of the unknown parameters are estimated from the data using linear functions. Such models are called “linear models.” Most commonly, linear regression refers to a model in of phenotype on the number of B alleles (0, 1, or 2), which is shown as the Linear Effect line. For any genotype, BiBj, the expected phenotype can then be written as the sum of the overall mean, a linear effect, and a dominance deviation:
- Pij = μ + αi + αj + dij = Population mean There are other statistical measures that use samples that some people confuse with averages - including 'median' and 'mode'. Other simple statistical analyses use measures of spread, such as range, interquartile range, or standard deviation. For a real-valued random variable X, the mean is the expectation of X. Note that not every probability + Additive Effect (aij = αi + αj) + Dominance Deviation (dij).
The additive genetic variance is the weighted average The weighted mean is similar to an arithmetic mean , where instead of each of the data points contributing equally to the final average, some data points contribute more than others. The notion of weighted mean plays a role in descriptive statistics and also occurs in a more general form in several other areas of mathematics of the squares of the additive effects:
where f(bb)abb + f(Bb)aBb + f(BB)aBB = 0.
There is a similar relationship for variance of dominance deviations:
where f(bb)dbb + f(Bb)dBb + f(BB)dBB = 0.
Narrow-sense heritability is defined as
and quantifies only the portion of the phenotypic variation that is additive (allelic) by nature (note upper case H2 for broad sense, lower case h2 for narrow sense). When interested in improving livestock via artificial selection Artificial selection describes intentional breeding for certain traits, or combination of traits. The term was utilized by Charles Darwin in contrast to natural selection, in which the differential reproduction of organisms with certain traits is attributed to improved survival or reproductive ability (“Darwinian fitness”). As opposed to, for example, knowing the narrow-sense heritability of the trait of interest will allow predicting how much the mean of the trait will increase in the next generation as a function of how much the mean of the selected parents differs from the mean of the population from which the selected parents were chosen. The observed response to selection leads to an estimate of the narrow-sense heritability (called realized heritability).
Estimating heritability
Estimating heritability is not a simple process, since only P can be observed or measured directly. Measuring the genetic Genetics , a discipline of biology, is the science of heredity and variation in living organisms. The fact that living things inherit traits from their parents has been used since prehistoric times to improve crop plants and animals through selective breeding. However, the modern science of genetics, which seeks to understand the process of and environmental variance requires various sophisticated statistical methods Statistics is the formal science of making effective use of numerical data relating to groups of individuals or experiments. It deals with all aspects of this, including not only the collection, analysis and interpretation of such data, but also the planning of the collection of data, in terms of the design of surveys and experiments. These methods give better estimates when using data from closely related individuals - such as brothers, sisters, parents and offspring, rather than from more distantly related ones. The standard error The standard error of a method of measurement or estimation is the standard deviation of the sampling distribution associated with the estimation method. The term may also be used to refer to an estimate of that standard deviation, derived from a particular sample used to compute the estimate for heritability estimates is generally very poor unless the dataset is large.
Figure 2. Heritability for nine psychological traits as estimated from twin studies Twin studies are one of a family of designs in behavior genetics which aid the study of individual differences by highlighting the role of environmental and genetic causes on behavior. All sources are twins raised together (sample size shown inside bars). MZ: Monozygotic twins, DZ: Dizygotic twinsIn non-human populations it is often possible to collect information in a controlled way. For example, among farm animals it is easy to arrange for a bull to produce offspring from a large number of cows. Due to ethical concerns, such a degree of experimental control When an experiment is conducted for the purpose of determining the effect of a single variable of interest on a particular system, a scientific control is used to minimize the unintended influence of other variables on the same system. Such extraneous variables include researcher bias, environmental changes, and biological variation. Scientific is impossible when gathering human data.
As a result, studies of human heritability sometimes contrast identical twins A twin is one of two offspring produced in the same pregnancy who have been separated early in life and raised in different environments (see for example Fig. 2). Such individuals have identical genotypes and can be used to separate the effects of genotype and environment.
Twin studies Twin studies are one of a family of designs in behavior genetics which aid the study of individual differences by highlighting the role of environmental and genetic causes on behavior entail problems of their own, such as: independently raised twins shared a common prenatal environment; they may have undergone intrauterine competition; the mother may be more physically stressed (less nutrients); and twins reared apart are difficult to find, and may reflect certain types of environments.
Heritability estimates are always relative to the genetic and environmental factors in the population, and are not absolute measurements of the contribution of genetic and environmental factors to a phenotype. Heritability estimates reflect the amount of variation in genotypic effects compared to variation in environmental effects.
Heritability can be made larger by diversifying the genetic background, e.g., by using only very outbred individuals (which increases the Variance(G)) and/or by minimizing environmental effects (which decreases the Variance(E)). Smaller heritability, on the other hand, can be generated by using inbred Inbreeding is the reproduction from the mating of two genetically related parents, which can increase the chances of offspring being affected by recessive or deleterious traits. This generally leads to a decreased fitness of a population, which is called inbreeding depression. Deleterious alleles causing inbreeding depression can subsequently be individuals (which decreases the Variance(G)) or individuals reared in very diverse environments (which increases the Variance(E)). Due to such effects, different populations of a species might have different heritabilities even for the same trait.
In observational studies In statistics, an observational study draws inferences about the possible effect of a treatment on subjects, where the assignment of subjects into a treated group versus a control group is outside the control of the investigator. This is in contrast with controlled experiments, such as randomized controlled trials, where each subject is randomly G and E may be correlated, giving rise to gene environment correlation Genetic factors influence exposure to many features of the environment. This comes about because people actively shape their experiences according to their personality and behavior, which are heritable. A consequence is that the relationship between environmental exposure and disease may be confounded by genotype. This has implications for. Depending on the methods used to estimate heritability, correlations between genetic factors and shared or non-shared environments may or may not be included in the total heritability estimate. [2]
Because of the contextual nature of measured heritabilities, paradoxes often arise. For example, the heritability of a trait could be near 100% in one study and close to zero in another. In one study, e.g., a group of unrelated army recruits may be given identical training and nutrition and then their muscular strength Physical strength, also know as Muscular strength, is the ability of a person to exert force on physical objects using muscles. Increasing physical strength is the goal of strength training may be measured.
The variation in strength observed after the (identical) training will translate into a high heritability estimate. In another study, whose purpose might be to assess the efficacy of various workout regimes or nutritional programs, study subjects may be first chosen to match each other as closely as possible in prior physical characteristics before some of them are put onto Program A and others onto Program B, and this will lead to a low heritability estimate.
Heritability estimates are often misinterpreted. Heritability refers to the proportion of variation between individuals in a population that is influenced by genetic factors. Heritability describes the population, not individuals within that population. For example, It is incorrect to say that since the heritability of a personality trait is about .6, that means that 60% of your personality is inherited from your parents and 40% comes from the environment.
The heritability estimate changes according to the genetic and environmental variability present in the population. In studies of genetically identical inbred animals, all traits have zero heritability. Heritability estimates can be much higher in outbred (genetically variable) populations under very homogeneous environments.
A highly genetically loaded trait (such as eye color Eye color is a polygenic phenotypic character and is determined by the amount and type of pigments in the eye's iris. Humans and other animals have many phenotypic variations in eye color, as blue, brown, gray, green and others. These variations constitute phenotypic traits) still assumes environmental input within normal limits (a certain range of temperature, oxygen in the atmosphere, etc.). A more useful distinction than "nature vs. nurture" is "obligate vs. facultative" -- under typical environmental ranges, what traits are more "obligate" (e.g., the nose -- everyone has a nose) or more "facultative" (sensitive to environmental variations, such as specific language learned during infancy). Another useful distinction is between traits that are likely to be adaptations (such as the nose) vs. those that are byproducts of adaptations (such the white color of bones), or are due to random variation (non-adaptive variation in, say, nose shape or size).
Estimation methods
There are essentially two schools of thought regarding estimation of heritability.
One school of thought was developed by Sewall Wright Sewall Green Wright was an American geneticist known for his influential work on evolutionary theory and also for his work on path analysis. With R. A. Fisher and J.B.S. Haldane, he was a founder of theoretical population genetics. He is the discoverer of the inbreeding coefficient and of methods of computing it in pedigrees. He extended this work at The University of Chicago The University of Chicago is a private, coeducational research university in Chicago, Illinois, USA. It was founded by the American Baptist Education Society, with a donation from oil magnate and philanthropist John D. Rockefeller. Incorporated in 1890, William Rainey Harper became the university's first president in 1891 and the first classes, and further popularized by C. C. Li (University of Chicago The University of Chicago is a private, coeducational research university in Chicago, Illinois, USA. It was founded by the American Baptist Education Society, with a donation from oil magnate and philanthropist John D. Rockefeller. Incorporated in 1890, William Rainey Harper became the university's first president in 1891 and the first classes) and J. L. Lush (Iowa State University Iowa State University of Science and Technology, more commonly known as Iowa State University , is a public land-grant and space-grant research university located in Ames, Iowa, United States. Iowa State is currently ranked by U.S. News & World Report as among the top 50 public universities and engineering schools in the United States. Iowa). It is based on the analysis of correlations and, by extension, regression. Path Analysis In statistics, path analysis is used to describe the directed dependencies among a set of variables. As such, models equivalent to any form of multiple regression analysis, factor analysis, canonical correlation analysis, discriminant analysis, as well as more general families of models in the multivariate analysis of variance and covariance was developed by Sewall Wright Sewall Green Wright was an American geneticist known for his influential work on evolutionary theory and also for his work on path analysis. With R. A. Fisher and J.B.S. Haldane, he was a founder of theoretical population genetics. He is the discoverer of the inbreeding coefficient and of methods of computing it in pedigrees. He extended this work as a way of estimating heritability.
The second was originally developed by R. A. Fisher Sir Ronald Aylmer Fisher FRS was an English statistician, evolutionary biologist, eugenicist and geneticist. He was described by Anders Hald as "a genius who almost single-handedly created the foundations for modern statistical science," and Richard Dawkins described him as "the greatest of Darwin's successors" and expanded at The University of Edinburgh The University of Edinburgh, founded in 1582, is an internationally renowned centre for teaching and research in Edinburgh, the capital city of Scotland. It was the fourth university to be established in Scotland, making it one of the ancient universities of the United Kingdom, Iowa State University Iowa State University of Science and Technology, more commonly known as Iowa State University , is a public land-grant and space-grant research university located in Ames, Iowa, United States. Iowa State is currently ranked by U.S. News & World Report as among the top 50 public universities and engineering schools in the United States. Iowa, and North Carolina State University North Carolina State University at Raleigh is a public, coeducational, extensive research university located in Raleigh, North Carolina, United States. Commonly known as NC State, NCSU, or simply "State" inside of North Carolina, the University is the principal technological institution of the University of North Carolina system, and is, as well as other schools. It is based on the analysis of variance In statistics, analysis of variance is a collection of statistical models, and their associated procedures, in which the observed variance is partitioned into components due to different sources of variation. In its simplest form ANOVA provides a statistical test of whether or not the means of several groups are all equal, and therefore of breeding studies, using the intraclass correlation of relatives. Various methods of estimating components of variance (and, hence, heritability) from ANOVA In statistics, analysis of variance is a collection of statistical models, and their associated procedures, in which the observed variance is partitioned into components due to different sources of variation. In its simplest form ANOVA provides a statistical test of whether or not the means of several groups are all equal, and therefore are used in these analyses.
Regression/correlation methods of estimation
The first school of estimation uses regression and correlation to estimate heritability.
Selection experiments
Figure 3. Strength of selection (S) and response to selection (R) in an artificial selection experiment, h2=R/S.Calculating the strength of selection, S (the difference in mean trait between the population as a whole and the selected parents of the next generation, also called the selection differential [3]) and response to selection R (the difference in offspring and whole parental generation mean trait) in an artificial selection experiment will allow calculation of realized heritability as the response to selection relative to the strength of selection, h2=R/S as in Fig. 3.
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