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In this article we will discuss about:- 1. Introduction to Polygenic Traits 2. Features of Polygenic Traits 3. Similarities between Oligogenic and Polygenic Traits 4. Analysis 5. Assumptions 6. Examples 7. Partitioning of Polygenic Variability 8. Significance of Polygenes.
Contents:
- Introduction to Polygenic Traits
- Features of Polygenic Traits
- Similarities between Oligogenic and Polygenic Traits
- Analysis of Polygenic Traits
- Assumptions of Polygenic Traits
- Examples of Polygenic Traits
- Partitioning of Polygenic Variability
- Significance of Polygenes
1. Introduction to Polygenic Traits:
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Character or trait refers to any property of an individual showing heritable variation. It includes morphological, physiological, biochemical and behavioural properties. Some characters are governed by one or few genes. Such traits are referred to as qualitative characters or oligogenic characters.
On the other hand, some characters are controlled by several genes. They are known as quantitative characters or polygenic characters. The mode of inheritance of polygenic characters is termed as polygenic inheritance or quantitative inheritance. Since in polygenic inheritance several genes (factors) are involved, it is also known as multiple factor inheritance.
2. Features of Polygenic Traits:
The term polygene was introduced by Mather in 1941. This term has found wide usage in quantitative genetics replacing the older term multiple gene.
Main features of polygenic characters are briefly presented below:
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1. Each polygenic character is controlled by several independent genes and each gene has cumulative effect.
2. Polygenic characters exhibit continuous variation rather than a discontinuous variation. Hence, they cannot be classified into clear-cut groups.
3. Effect of individual gene is not easily detectable in case of polygenic characters and, therefore, such traits are also known as minor gene characters.
4. The statistical analysis of polygenic variation is based on means, variances and co-variances, whereas the discontinuous variation is analysed with the help of frequencies and ratios. Thus, polygenic characters are studied in quantitative genetics and oligogenic characters in mendelian genetics.
5. Polygenic traits are highly sensitive to environmental changes, whereas oligogenic characters are little influenced by environmental variation.
6. Classification of polygenic characters into different clear-cut groups is not possible because of continuous variation from one extreme to the other. In case of qualitative characters, such grouping is possible because of discrete or discontinuous variation.
7. Generally the expression of polygenic characters is governed by additive gene action, but now cases are known where polygenic characters are governed by dominance and epistatic gene action. In case of oligogenic characters, the gene action is primarily of non-additive type (dominance and epistasis).
8. In case of polygenic characters, metric measurements like size, weight, duration, strength, etc. are possible, whereas in case of oligogenic characters only the counting of plants with regard to various kinds like colour and shape is possible. Thus, metric measurement is not possible in case of oligogenic characters.
9. Transgressive segregants are only possible from the crosses between two parents with mean values for a polygenic character. Such segregants are not possible in case of qualitative or oligogenic traits.
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10. The transmission of polygenic characters is generally low because of high amount of environmental variation. On the other hand, oligogenic characters exhibit high transmission because there is little difference between the genotype and phenotype of such character. Thus, polygenic characters differ from oligogenic ones in several aspects (Table 12.1).
In plant breeding both types of characters showing qualitative and quantitative inheritance have equal economic importance.
3. Similarities between Oligogenic and Polygenic Traits:
East (1916) demonstrated that polygenic characters were perfectly in agreement with Mendelian segregation and later on Fisher (1918) and Wright (1921, 1935) provided a mathematical basis for the genetic interpretation of such characters.
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The quantitative characters do not differ in any essential feature from the qualitative characters, as discussed below:
1. Both quantitative and qualitative characters are governed by genes; the former is controlled by polygenes or minor genes and the latter by oligogenes or major genes.
2. Both major as well as minor genes are located on the chromosome in the nucleus.
3. The polygenic traits controlling continuous variation exhibit segregation like major genes controlling discontinuous Mendelian variation.
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4. Polygenic characters show variable expression which is due to non-genetic causes i.e., environmental effects. Qualitative characters also exhibit variation in expression but to a lesser degree than polygenic traits.
5. The reciprocal crosses for both types of traits exhibit close agreement in expression of genes.
6. The phenomenon of transgression in polygenes can only be explained by Mendelian principles of inheritance.
7. Polygenes mutate like oligogenes.
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8. Dominance and non-allelic interactions are common features of major genes. These features are also observed for polygenes, but are usually complete for major genes and only partial for minor genes.
9. Polygenes exhibit linkage like oligogenes. Many cases of linkage between major genes and polygenes controlling continuous variation have been reported.
Thus, quantitative genetics or biometrical genetics is an extension of Mendelian genetics firmly based on Mendelian principles of heredity.
4. Analysis of Polygenic Traits:
The method of analysis of quantitative inheritance differs from that of qualitative inheritance in some aspects as given below:
1. It requires various measurements of characters like weight, length, width, height, duration, etc., rather than classification of individuals into groups based on colour or shape.
2. Observations are recorded on several individuals and the mean values are used for genetical studies. Segregation into distinct classes in F2 generation-is not obtained in the inheritance of quantitative characters. The segregants exhibit continuous range of variation from one extreme (low) to other (high) for such traits.
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3. The inheritance is studied with the help of mean, variances and covariance’s. These estimates can be worked out from data recorded in replicated experiment.
4. Fisher (1918) was the pioneer worker to interpret the quantitative characters in terms of Mendelian genetics. Now several biometrical techniques are available for the genetic analysis of quantitative characters. The science which deals with the genetic interpretations of quantitative characters has got separate entity as quantitative genetics or biometrical genetics.
5. Assumptions of Polygenic Traits:
Polygenic inheritance is based on several assumptions.
The six important assumptions are given below:
1. Each of the contributing genes involved in the expression of a character produces an equal effect.
2. Each contributing allele has either cumulative or additive effect in the expression of a character.
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3. The genes involved in the expression of characters have lack of dominance. They show intermediate expression between two parents.
4. There is no epistasis among genes at different loci.
5. The linkage is in equilibrium, means there is no linkage.
6. The environmental effects are absent or may be ignored. However, last three assumptions are seldom fulfilled.
There are two types of alleles or genes in the polygenic inheritance, viz:
(1) Contributing alleles and
(2) Non-contributing alleles.
Those alleles which contribute to continuous variation are known as contributing alleles and those which do not contribute to continuous variation are referred to as non-contributing alleles. Some scientists refer to these as effective and non-effective alleles, respectively.
6. Examples of Polygenic Traits:
In plant genetics, examples of polygenic characters include yield per plant, days to flower, days to maturity, seed size, seed oil content, etc. Examples of qualitative characters are colour of stem, flower, pollen, etc. and their shapes.
Polygenic inheritance has been reported for various characters both in plants and animals. The most common examples include kernel colour in wheat, corolla length in tobacco, skin colour in man and ear size in maize.
These are briefly described as follows:
1. Kernel Colour in Wheat:
Nilsson Ehle (1908) studied the inheritance of kernel colour in wheat. He found that seed or kernel colour in wheat is governed by one, two and three gene pairs, because in the crosses between red and white kernel varieties, he observed that the F1 was intermediate between the parental values and in F2 he observed 3:1,15:1 and 63 : 1 ratios of red and white seeds in different crosses.
The last two ratios indicated that there was duplicate gene interaction; however, in depth study of coloured seeds revealed that there were different grades or shades of colour within the red coloured seeds. The red seeds of 15 : 1 ratio could be easily divided into four classes on the basis of shade of colour, viz., dark red, medium dark red, medium red and light red.
These colours were observed in the ratio of 1 : 4 : 6 : 4 : 1. This suggested that the seed colour in wheat is controlled by genes which show lack of dominance and have small cumulative effects.
Here, two types of alleles are involved in the expression of character. Those which contribute to continuous variation and those which do not contribute. The first category of alleles is called effective and second as non-effective. Assume that red seed colour is controlled by two genes R1 and R2 and, white seed colour by r1 and r2.
From the cross between dark red and white seed parents, Nilsson Ehle observed the following results (Fig. 12.1):
Where 4 effective alleles were present, the seed colour was dark red, where 3 such alleles were present, the seed colour was medium dark red, with 2 effective alleles, colour was medium red and with 1 effective allele, seed colour was light red. White seed colour was produced when all the non-effective alleles were present.
2. Corolla Length in Tobacco:
Extreme differences exist in corolla length in Nicotiana longiflora. East (1916) studied the inheritance of corolla length in this species of tobacco. He crossed inbred lines of this species with average corolla length of 40 cm and 93 cm.
The F1 showed intermediate expression for corolla length with 63 cm. In F2, wide variation for corolla length was observed. The results indicated that five or more genes were involved in the expression of corolla length.
3. Skin Colour Inheritance in Man:
The inheritance of skin colour in man was studied by Davenport. The inheritance of Negro x white matings can be explained on the basis of two gene difference. Assume that negro colour is governed by A and B genes and white colour by a and b genes.
A cross between negro and white gives birth to a child with medium skin colour called mullatoes (F1). In F2 generation, four distinct shades of black colour were observed besides one white (Fig. 12.2). Thus, the phenotypic ratio of 1 : 4 : 6 : 4 : 1 was observed. The individuals having 4, 3, 2, 1 and 0 effective alleles had black (negro) dark, medium, light and white colour, respectively.
The results are presented below:
Subsequent studies on skin colour inheritance indicated that as many as six genes are involved in the expression of this character.
Transgressive Segregation:
Appearance of transgressive segregants in F2 is an important feature of polygenic inheritance. Segregants which fall outside the limits of both the parents are known as transgressive segregants. Transgressive segregation results due to fixation of dominant and recessive genes in separate individuals.
Such segregation occurs when the parents are intermediate to the extreme values of the segregating population. Plant breeders use this principle to obtain superior combinations in segregating material for polygenic characters.
An example of transgressive segregation is presented as follows:
Environmental Effect:
Polygenic characters are highly sensitive to environmental changes. In other words, they are more prone to genotype x environmental interactions. The main effect of environment is to mask the small differences among different genotypes resulting in continuous variation in the character.
When the contribution of environment is 50 per cent, the distribution becomes roughly similar to normal curve and with 75 per cent contribution, it tends to reach normal distribution. For polygenic traits, generally the environmental variation ranges from 10 to 50 per cent and even more for some traits like yield. The high environmental variation results in overlapping of various classes resulting in continuous, variation.
7. Partitioning of Polygenic Variability:
The polygenic variation or variability present in a genetic population is measured in terms of variances.
The polygenic variation is of three types, viz:
(1) Phenotypic,
(2) Genotypic and
(3) Environmental.
These are briefly described below:
1. Phenotypic Variability:
It is the total variability which is observable. It includes both genotypic and environmental variation and hence changes under different environmental conditions. Such variation is measured in terms of phenotypic variance.
2. Genotypic Variability:
It is the inherent or genetic variability which remains unaltered by environmental conditions. This type of variability is more useful to a plant breeder for exploitation in selection or hybridization. Such variation is measured in terms of genotypic variance. The genotypic variance consists of additive, dominance and epistatic components.
3. Environmental Variability:
It refers to non-heritable variation which is entirely due to environmental effects and varies under different environmental conditions. This uncontrolled variation is measured in terms of error mean variance. The variation in true breeding parental lines and their F1 is non-heritable. Fisher was the first to divide in 1918, the genetic variance into additive, dominance and epistatic components.
a. Additive Variance:
It refers to that portion of genetic variance which is produced by the deviations due to average effects of genes at all segregating loci. Thus, it is the component which arises from differences between two homozygotes of a gene, i.e., AA and aa. Additive genes show lack of dominance, i.e., intermediate expression.
The additive genetic variance is associated with homozygosis and, therefore, it is expected to be maximum in self-pollinating crops and minimum in cross-pollinating crops. Additive variance is fixable and, therefore, selection for traits governed by such variance is very effective.
Additive genetic variance is important for the following major reasons:
1. It is required for estimation of heritability in narrow sense and response to selection is directly proportionate to narrow sense heritability.
2. It is a pre-requisite for selection because this is the only variance which responds to selection.
3. Breeding value of an individual is measured directly by the additive gene effects. The general combining ability (gca) effect of a parent is measure of additive gene effects.
4. Additive genetic variance gets depleted proportionate to the improvement made by selection.
5. In natural plant breeding populations, additive variance is the predominant one closely followed by dominance variance.
b. Dominance Variance:
It arises due to the deviation from the additive scheme of gene action resulting from intra-allelic interaction i.e., interaction between alleles of the same gene or same locus. It is due to the deviation of heterozygote (Aa) from the average of two homozygotes (AA and aa).
Such genes show incomplete, complete or over-dominance. The dominance variance is associated with heterozygosis and, therefore, it is expected to be maximum in cross-pollinating crops and minimum in self-pollinating species.
Dominance variance is not fixable and, therefore, selection for traits controlled by such variance is not effective. Heterosis breeding may be rewarding in such situation. Dominance variance differs from additive variance in several ways. (Table 12.2).
c. Epistatic Variance:
It arises due to the deviation as a consequence of inter-allelic interaction, i.e., interaction between alleles of two or more different genes or loci. The epistatic variance is of three types, viz., (i) additive x additive, (ii) additive x dominance, and (iii) dominance x dominance. They differ from each other in several aspects. (Table 12.3).
(i) Additive x Additive:
In this case both the interacting loci exhibit lack of dominance individually. It is denoted as A x A and is fixable.
(ii) Additive x Dominance:
It refers to interaction between two or more loci, one exhibiting lack of dominance and the other dominance individually. It is denoted as A x D and is non-fixable.
(iii) Dominance x Dominance:
In this type of epistasis both the interacting loci exhibit dominance individually. It is represented as D x D and is non-fixable.
The first type of epistasis is fixable and, therefore, selection is effective for traits governed by such variance. The last two types of epistatic variances are unfixable and, therefore, heterosis breeding may be rewarding for traits exhibiting such variance. In natural plant breeding populations, epistatic variance has the lowest magnitude. Epistatic variance differs in many aspects from dominance variance.
Wright (1935) suggested the partitioning of genetic variance into two components, viz., additive and non-additive (dominance and epistatic components), of which only the additive component contributes to genetic advance under selection.
Mather (1949) divided the phenotypic variance into three components, namely, (1) heritable fixable (additive variance), (2) heritable non-fixable (dominance and epistatic components), and (3) non-heritable non-fixable (Environmental fraction).
In fact, the heritable fixable component of phenotypic variance will include the additive x additive fraction of the epistatic variance as well. Further, the total phenotypic variance may be partitioned as (1) fixable (additive and additive x additive components) and (2) non-fixable (dominance, additive x dominance and dominance x dominance types of epistasis and environmental fraction) components.
The above discussion may be summarized as follows:
VP = VG + VE; VG = VA + VD + VI; and VI = VAA + VAD + VDD
Where; VP = phenotypic variance, VG = genotypic variance, VA = additive variance, VD = dominance variance, VI = epistatic variance, VAA = additive x additive variance, VAD = additive x dominance variance, and VDD = dominance x dominance variance.
In homozygous genotypes, the genetic variance is of additive (A) and additive epistatic (AA) types, while in the segregating populations all the three types of genetic variances, viz., additive, dominance and epistasis are observed. In F2, the phenotypic variance has 1/2D (additive) and 1/4H (non-additive) components.
In a random mating populations with no epistasis and zero inbreeding, the covariance between a parent and its offspring is 1/2 VA; the covariance among half-sibs is 1/4 VA; and the covariance among full-sibs is 1/2 VA + 1/4 VD. These relationships change with the level of inbreeding in the population.
Genetic variability for important agronomic traits in almost all the crops is mainly due to the additive genetic variance. The non-additive variance also exists in nearly all crops and for many important traits, but it is generally smaller in magnitude than the additive component.
The variability present in genetic populations can be assessed in four different ways: (1) using simple measures of variability, (2) by variance component analysis, (3) by D2 statistics, and (4) by metro glyph analysis. For details of these procedures refer Singh and Narayanan (1993).
8. Significance of Polygenes:
Polygenes are of prime importance to plant breeder for evolution of improved cultivars. Polygenes have great evolutionary significance. They provide variation of fine adjustment and are systems of smooth adaptive change and of speciation.
The potential genetic variability is stored in the form of linked polygenic complexes. Such stores bear mixtures of plus and minus alleles. The potential or hidden variability is released, after inter-mating of such genotypes with other genotypes, due to segregation and recombination.
Mather (1943) has nicely explained the mechanism of storage and release of polygenic variability. It is believed that in natural populations, the best adapted or the fit individuals are those that are close to the population mean for various quantitative traits.
Mather recognized two types of variability, viz:
(1) Free variability and
(2) Potential variability.
1. Free Variability:
It refers to phenotypic differences between homozygotes with extreme phenotypes. Such variability is expressed and exposed to selection. Natural selection acts against extreme phenotypes.
2. Potential Variability:
If refers to hidden or bound variability in the heterozygotes or in the homozygotes which do not have the extreme phenotype and, therefore, is not exposed to selection.
It is of two types as given below:
Heterozygotic Potential Variability:
This type of variability is stored in heterozygote, e.g., AaBb. Such heterozygotes are phenotypically uniform and are very close to the population mean. However, they would produce extreme phenotypes in the next generation due to segregation and recombination. Thus, the heterozygotes function as stores of variability which is released slowly as free variability due to segregation and recombination.
Homozygotic Potential Variability:
Homozygotes also function as stores of variability. For example, two gene homozygotes AAbb and aaBB may be expected to cluster around the mean of the population. They would, therefore, be protected from natural selection and would be phenotypically uniform.
However, they would produce the extreme phenotypes AABB and aabb after crossing, i.e., AAbb x aaBB followed by segregation and recombination. The release of this type of variability is slow because it must first be converted into heterozygotic potential variability through hybridization and then it is released as free variability.
In case of polygenic traits, several genes governing a character may be present on the same chromosome. It would be advantageous to the population if these genes were linked in the repulsion phase, i.e., some dominant genes were linked with some recessive genes.
For example, out of the three schemes for the arrangement of four genes, A, B, C, and D, given below, scheme number three would be the most desirable. Because in this scheme, the full release of variability would require three crossovers at precise points (marked x).
It may be expected that natural populations would develop such complex and elaborate gene arrangement for storing variability. This would permit them to meet the opposing demand of immediate fitness and long term evolutionary requirements.
This mechanism of storage and release of genetic variability in the form of polygenic complexes gives response to selection in new direction. The linkage among polygenes is useful. It reduces immediate response to selection but prolongs the response to selection due to slow release of potential genetic variability in the segregating generations.
