In 1909, Swedish geneticist H. Nilsson Heller put forward the polygene theory, which used Mendelian segregation of each pair of minor genes to explain the inheritance of quantitative traits.British statistician and geneticist RA.FisherAmerican geneticist S. Wright and British physiologist and geneticist JB. S. Haldane laid the theoretical foundation of quantitative genetics in the 1920s.In the 1940s, the American scholar J50. Rush and K. Mather, a British quantitative geneticist, further developed the study of quantitative genetics, and K. Mather called it geneticism.Since the 1950s, along withprobability theory、linear algebra、Multivariate statisticsThe gradual application of quantitative genetics and stochastic processes has made great progress in the content of quantitative genetics.
In 1909, Swedish geneticist H. Nilsson Heller put forward the polygene theory, which used Mendelian segregation of each pair of minor genes to explain the inheritance of quantitative traits.British statistician and geneticist RA.FisherAmerican geneticist S. Wright and British physiologist and geneticist JB. S. Haldane laid the theoretical foundation of quantitative genetics in the 1920s.In the 1940s, the American scholar J50. Rush and K. Mather, a British quantitative geneticist, further developed the study of quantitative genetics, and K. Mather called it geneticism.Since the 1950s, along withprobability theory、linear algebra、Multivariate statisticsThe gradual application of quantitative genetics and stochastic processes has made great progress in the content of quantitative genetics.[1]
Research methods and contents
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Mainly usedBiostatisticsThe method has a certain effect on the groupQuantitative traitCarry out random sampling measurement and calculateaverage、varianceAnd mathematical analysis on this basis.According to Danish plant physiologist and geneticist W50. Johansson's study specified quantitative traitsPhenotypic valueP is equal to the sum of genotype value G and environment value E;The average phenotypic value of the population is equal to the strong average genotype value (because the ∑ E of the population is 0);The genotype value is determined by the cumulative effect value A, dominant effect value D of genes and theEpistasisEffect value I composition.Thus, for a quantitative trait of a populationgenetic variationThat is, it can be represented by the genetic variance VG, which is the cumulative variance VADominant varianceThe sum of VD and epistatic variance VI.If the interaction between environmental factors and genetic factors is not considered, the measuredPhenotypic variationOfPhenotypic variance(VP) is equal to the genetic variance VG andEnvironmental varianceThe sum of VE can be written as the following formula:
VP=VG+VE=VA+VD+VI+VE According to this formula, as long as the environmental variance can be estimated (for example, it can be expressed by the variance of the pure line parent or the hybrid generation), the size of the genotypic variance in the representative variance of the segregation generation of the hybrid can be measured.On this basis, a series of instructive genetic parameters in breeding practice can also be estimated, such asHeritabilityRepetitive forcegenetic correlation 、Genetic progressAnd selection index, etc.These parameters can be used to analyze and predictQuantitative traitThe genetic dynamics of variation can be used as a reference for animal and plant breeding.
Another important aspect of quantitative genetics is the study of various genetic mating designs (e.gDiallel cross, reincarnation selection, various kinds of animalsMating systemAnd the genetic dynamics of quantitative traits in these mating designs.In addition,genotypeThe interaction with environment is also an important research topic of quantitative genetics in recent years.[1]
Correlation calculation
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Total variance P, variance Q of homozygous inbred lines, narrow heritability h2, environmental variance E, dominant genetic variance D, additive genetic variance A, broad heritability H2,Total genetic variance G, then E=Q/P=A;According to h2=D/(D+E);A=P-E-D；H2=（A+D）/（A+D+E）；G=P-E。[1]
Related concepts
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Heritability
Heritability, or heritability, is used to measure the proportion of a certain variation caused by genetic reasons (in terms of environmental impact) in the phenotypic variation in a population, which is used as a reference index for selection, so as to judge the possible degree of the variation of this trait to be transmitted to offspring. For example, the heritability of the birth weight of cows is 49% (see table),It shows that birth weight is largely determined by genetic factors, so it is more likely to succeed in breeding large cattle based on birth weight.On the contrary, the heritability of the pregnancy rate is 3%, which means that most of the pregnancy rate is not determined by genetic factors, so it is very unlikely to succeed in breeding prolific cows according to the pregnancy rate.Generalized heritability (h2B) is expressed as the ratio of genotypic variance to phenotypic variance (h2B=VG/VP).If the heritability is large, the variation of this character is mainly from genetic factors, and is less affected by environmental changes.Because only the cumulative variance VA is the variable that can be inherited from generation to generation, the ratio of cumulative variance to phenotypic variance is often used in breeding to represent heritability (h2N=VA/VP), which is narrow heritability.Heritability is widely used in animal and plant breeding.Breeding practice often requiresQuantitative traitThe heritability of is used as a reference to determine the selection method and period, as well as to predict the selection effect and estimate the cumulative effect value, i.e. breeding value.
Repetitive force
Repeatability refers to an individual's certain degree ofQuantitative traitOfPhenotypic valueThe degree of possible repetition, used to measure the degree ofgenotypeStability expressed in a fluctuating environment;It can also be used to study the approximation of a quantitative trait in a population in different environments.Repetitive force is alsoIntra group correlation coefficientTherefore, it can also determine the number of times a certain phenotypic value should be measured.For example, the repeatability of milk fat rate is 80%, which indicates that the repeatability is high, and the future milk fat rate level of the cow can be roughly determined by measuring a few times.In addition, repeatability can also be used to estimate the stability of a trait in a population or individual.
genetic correlation
Genetic correlation refers to two traits of the same individualgenotypeThe correlation between the cumulative effects inGenetic covarianceInherited with various charactersstandard deviationThe ratio of the product.Genetic correlation can reflect the degree of correlation between genotypes, so we can use the traits with high heritability toIndirect selectionSome economic traits with high genetic correlation but low heritability or difficult to measure, so as to improve the selection effect.
becausegenetic correlation The environmental impact has been removed, so it is more thanPhenotypic correlationReliable.For example, the heritability of egg weight variation in domestic chickens is 60%, and that of body weight variation is only 31%;This suggests that the effect of breeding large breeders by weight is not good. Although the phenotypic correlation of these two traits is only 0.16%, the genetic correlation is up to 50%. Therefore, large breeders can be indirectly bred by egg weight traits.Another example is that there is a close genetic relationship between the cow's milk yield and the cow's body shape and breast shape, and the rice yield is the same asYield factor(such as panicles per plant, grains per panicle and 1000 grain weight).
In breeding, the selection value of one trait is usually predicted indirectly from the characteristics of another trait according to the correlation of traits.However, this is only phenotypic correlation, which also includes environmental impact, so it cannot truly reflect the genetic relationship between different traits.Real reflection of genetic correlationgenotypeWe can get better results based on the correlation betweenHeritabilityThe lower character has more significant effect.
Genetic progress
Genetic progress (also known asGenetic availability）It's a hybrid offspringQuantitative traitOfaverageAt a certain timeSelect intensityBelow is the value higher than the original population average.It is a function of heritability h2 and selection difference i, namely Δ G=ih2.Selection differenceIt refers to the difference between the average value of a quantitative trait in the population and the average value of the trait selected as the next generation parent.Genetic progress is an important estimate for determining genetic effects.Just find the amphotericHeritabilityAnd between themgenetic correlation Can be estimated by selecting a characterIndirect selectionRelated genetic progress of another trait.
Genetic progressAccording to the heritability of traits, there are different effects.For high heritabilityGenetic varianceFor large traits, larger genetic progress can be obtained under certain selection intensity, indicating that the selection effect of this trait is higher.Therefore, genetic progress is an important parameter to determine the selection effect.In addition, the relationship of genetic progress among characters is the same as that between characters. The selection of one character will affect the genetic progress of another character.The correlation between the two characters only relatively indicates the closeness of the two characters;The correlation of genetic progress between two traits indicates the relationship between the absolute values of two traits in heredity.Therefore, genetic progress can be used not only to predict the absolute progress of a trait under selection, but also to predict the corresponding progress caused by other traits.Therefore, we can make appropriate evaluation on breeding materials, and make reasonable arrangement and treatment for the tested materials.
Selection index
The selection index is for multipleQuantitative traitComprehensive selectionSelect Indicator, can be used toTarget traitsThe greatest improvement is achieved through selection.The selection index value is equal to each characterPhenotypic valueThe algebraic sum of the product with the exponential coefficient.As the nature and requirements of the problem are different, the methods for determining the index coefficient are also different, so there are many selection indexes of different estimation methods that can be used as indicators for the selection of multiple quantitative traits.[1]
Disciplinary significance
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Genetic laws andBiostatisticsAnd other branches of mathematicsQuantitative traitThe laws of genetics and biological development enrich and enrich the theory of genetics and evolution.Since most economic traits are quantitative traitsgenetic variationIt plays an important guiding role in breeding practice.[1]
