POLYGENIC INHERITANCE/MULTIPLE GENE INHERITANCE QUANTITATIVE INHERITANCE

POLYGENIC INHERITANCE/MULTIPLE GENE INHERITANCE QUANTITATIVE INHERITANCE

17.   POLYGENIC INHERITANCE/MULTIPLE GENE INHERITANCE QUANTITATIVE INHERITANCE

A polygene is a collection of non-epistatic genes that together effects a phenotypic trait. Polygenic inheritance occurs when one trait is controlled by two or more genes. Hence, we can say that genes are large in quantity but with small effect. The actual loci of non-allelic genes are unknown till date. Advances in statistical methodology and sequencing allow biologists to locate genes for that particular trait. These genes also show pleiotropy. Pleiotropy is when one gene influences two or more undulated phenotypic traits. If there is a mutation in a pleiotropic gene, it may have an effect on many traits simultaneously due to the gene coding for a product used by different targets that shows the same signalling function. The term 'pleiotropy' was coined by Ludwig plate in 1910.
Polygenic locus is a locus which included in the gene system responsible for a genetic component of variation in a quantitative character. Polygenic locus may be either single or complex i.e. either a single gene or closely linked many genes.
Examples of polygenic inheritance are height, skin colour, eye colour, weight etc. in humans. Polygenes are also present in drosophila, for example, wing morphology, bristle count etc.
In polygenic inheritance, a bell-shape curve is obtained which shows that more genes involved.


F2  generation :

Earlier, polygene mapping requires statistical tools to measure the effect of polygenes. One of these tools is QTL mapping (quantitative Trait loci). QTL is a part of DNA that correlates with variation in a phenotype. Gene which controls the phenotype is linked by QTL. QTL are mapped by identifying, molecular which markers, correlate with an observed trait. Quantitative traits are the product of two or many genes.
17.1.    Heritability
Heritability is a statistic used in genetics and breeding which estimates how much variation in a phenotypic trait because of genetic variation among individuals in the population. Environmental factors also lead to variation. Heritability is estimated by comparing the phenotypic variation of individuals among related individuals in a population. In quantitative genetics, especially in selective breeding and behaviour genetics, heritability plays an important role.
Heritability measures the fraction of phenotype variability that can be marked to genetic variation.
17.1.1.    Realized Heritability
Realized heritability is an estimate of true heritability in quantitative genetics. Realized heritability is observed in drosophila.
            H    =    Yo   –  Y           
                         Yp  –  Y
            H    =    Heritability
            Yo    =    Offspring yield
            Y    =    mean yield of the population
            Yp    =    Parental yield

Realized heritability is the gain divided by the selection differential when offspring are produced by parents with a mean that differs from that of the population.
In Drosophila number of bristles is also under polygenic control. In a population of drosophila, if the mean bristle number was 6.6. Three pairs of flies are used as parents; has a mean of 7.4 bristles.  Their offspring had a mean of 6.8 bristles. Hence, Yo  =  6.8,  Y  =  6.4, and Yp  =  7.4. Then dividing the gain by the selection differential.
        H    =    6.8  –  6.6    =    0.2    =    0.25
                     7.4  –  6.6          0.8    
17.1.2.    True Heritability
Variation in a trait can be separated into genetic and environment component. In true heritability, much of the variation can be attributed to genetic factors and have less impact of environmental factors.
True heritability is divided into two types :
1.    Broad  -  sense heritability
2.    Narrow  -  sense  heritability
17.1.3.    Broad - Sense Heritability
Variation observed among individuals of the same species under different environmental conditions that have arisen due to the non genetic factor. The genetic and environmental factors is assessed by the heritability index (H2). Heritability index is calculated by using an analysis of variance (V) among individuals of known genetic relationship. Heritability index is also called broad-sense heritability (H2) and it measures the degree to variation in genetic factors (VG) for a single population under control of environmental variation (VE).
Phenotypic variation is the sum of three components; environmental variance (VE or s2e), genetic or genotypic Variance (VG or s2g) and variance resulting from the G - E interaction or interaction of genetics and environment (VGE).
Phenotypic (VP)    =    VE  +  VG  +  VGE
Variance
VaE is usually negligible.
VP    =    VE  +  VG
Broad-sense heritability expresses that proportion of variance due to the genetic component.
            H2    =    VG
                           VP  
High heritability    >    0.5
Medium heritability    =    0.2  –  0
Low heritability    <    0.2
17.1.4.    Narrow-sense Heritability
Information related to heritability is most useful in animal & plant breeding as a measure of potential response to selection. In the case of artificial selection, a different estimate of heritability is used. Such an estimate is based on a subcomponent of VG referred to as additive variance (VA) :
        VG    =    VA    +    VD    +    VI
Here, VA represents additive variance which results from the average effect of the additive component of genes. Dominance variation, VD is the deviation from the additive components that results when the phenotypic expression in heterozygotes is not an intermediate between two homozygotes. Interactive variance, VI is deviation of two or more loci which are behaving epistatically of additive component. Interactive variance is often negligible. When VG is divided into VA and VD, a new heritability, (H2), or narrow-sense heritability can be calculated. Narrow-sense heritability is defined as a fraction of phenotypic variance shows a resemblance between parents and offsprings.
        h2    =    VA
                      VP 
    Because,  VP  =  VE  +  VA  +  VD                    h2    =    VA       
                                VE  +  VA  +  VD


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