Quantitative Genetics: Decoding the Complexity of Traits
- 1. Group 4 M.Sc Biotechnology SEM- 1 Department of biotechnology Guru Ghasidas Vishwavidyalaya, Koni, Bilaspur (C.G.) 495009 QUANTITATIVE GENETICS: HERITABILITY AND ITS MEASUREMENT SUBMMITEDBY;- EhteshamRaza FirdousAshraf Khileshpatel Khushboo Mahesh Kumari suman GUIDED BY ;- Dr. Ashish Kumar (Associate Professor) Department of Biotechnology Guru Ghasidas Vishwavidylaya, Bilaspur
- 2. INTRODUCTION OF QUANTITATIVE INHERITANCE •Quantitative inheritance is also known as multifactorial inheritance, polygenic, multiple gene, polygenic trait, complex trait inheritance. •When more than one gene determines the expression of particular trait, it is termed as Quantitative Genetics. • Dominant allele have cumulative effect. •There can be a large influence of environmental factor.
- 3. QUANTIATIVE TRAITS: •They are control by many genes (polygenic). •Each contributing allele has a small and relatively equal effect. The effect of each allele is additive. •It is affected by environment . •They show continuous variations.
- 4. QUALITATIVE TRAITS:-1. controlled by few genes 2.discontinuous variation 3. low environmental effect 4. ex- monohybride cross QUANTITATIVE TRAITS:-1. controlled by many genes 2. continuous variation 3. high environmental effect 4. ex- skin colour DIFFERENCE B/W QUALITATIVE & QUANTITATIVE GENETICS
- 6. POLYGENIC INHERITANCE •When one phenotypic character controlled by more than one gene it is called polygenic inheritance •The quantity of inheritance depends on dominant alleles. • Polygenic inheritance don't follow the mendelian ratio Example - skin colour , kernel colour of wheat , height etc
- 7. CHARACTERISTIC OF POLYGENIC INHERITANCE 1.Multiple genes: polygenic traits are controlled by two or more genes, each contributing to the phenotype. 2.Continuous variation: traits exhibit a range of continuous variation rather than distinct categories. For example, height or skin color can vary along a spectrum. 3. Additive effects: each gene involved adds a small amount to the overall phenotype,the more contributing genes, the more continuous variation 4. Environmental influence: environmental factor also impact the expression of polygenic treats, increasing the final phenotype. 5.Bell shape continuation: phenotypes trends follow the bell shape curve Fig: Skin color in human
- 8. Polyphonic inheritance in plants includes the colour and shape of the stem, pollen, flower, content, size of the seed, 1.Kernel colour of the wheat: ●H Nilsson-Ehle in 1909 crossed two wheat strains,one with red-kernel grain and other with white kernel grain that yielded plants in the F1 generation with grains of intermediate colour. When F1 generation was self fertilised, he observed seven kernel colour classes from red to white were distinguishable in a ratio of 1:4:6:4:1. POLYGENIC I NHERITANCE IN PLANTS
- 9. V
- 10. SKIN COLOUR IN HUMAN:- Inheritance of skin pigmentation is polygenic inheritance. Around 60 loci contribute to the inheritance of a single trait. If we take an example of a pair of alleles of three different and unlinked loci as A and a, B and b, C and c. The capital letters represent the incompletely dominant allele for dark skin colour. The more capital letters show skin colour towards the darker range and small letters towards the lighter colour of the skin. Parents having genotype AABBCC and aabbcc will produce offspring of intermediate colour generation I,e. AaBbCc in the F1 generation In F2 generation of two triple heterozygotes (AaBbCc × AaBbCc) mate, they will give rise to varying phenotypes ranging from very dark to very light in the ratio 1:6:15:20:15:6:1.
- 12. HERITABILITY
- 13. HERITABILITY DEFINITION: Heredity is the passing on of traits from parents to offspring, and heritability is the measure of how much individual differences in a trait within a population can be attributed to genetic factors. It's essentially the extent to which genetics influence the variation of a specific trait. Example: If the heritability of intelligence is 0.8, it means that 80% of the variation in intelligence among individuals is due to genetic factors, while the remaining 20% is influenced by other factors like environment and experiences. Phenotypic variance: it is the total range of observable differences in traits among individuals in a group taking into account both genetic and environmental factors. It is represented by Vp. COMPONENTS OF PHENOTYPIC VARIANCE Genotypic variance – it is inherent or genetic variability that is unaltered by the environment It is most useful for the breeders. And it is measuredin term of genetic variance •Environmental variance includes differences that result from environmental factors such as the amount of light or water that the plant receives; it also includes random differences in development that cannot be attributed to any specific factor. Any variation in phenotype that is not inherited is, by definition, a part of the environmental variance. •Third, Genotype-Environment Interaction: •Genotype-environmental interaction refers to the phenomenon where the expression of a genotype is influenced by the specific environment which it is situated.
- 14. • Genetic variance can be further dissected into three main components: •Additive Genetic Variance (VA): It comprises the additive effect of genes on the phenotype which can be summed to determine the overall an phenotype. •Dominance Genetic Variance (VD): Dominance effects occur when the interaction between alleles at a single gene influences the phenotype. This component accounts for the variability caused by dominant and recessive gene interactions. •Epistasis Genetic Variance (VI): Epistasis involves the interaction between genes at different loci. Epistasis genetic variance captures the contribution of these gene- gene interactions to the overall genetic variability observed in a population COMPONENTS OF GENETIC VARIANCE
- 15. COMPONENTS OF PHENOTYPIC VARIANCE
- 16. TYPES OF HERITIBILITY Heritability can be broadly classified into two type 1.Broad-Sense Heritability (H²) 2.Narrow-Sense Heritability (h²) Definition: Broad-sense heritability accounts for the total genetic variance influencing a trait within a population. Components: The broad-sense heritability is the portion of phenotypic variance that is due to all types of genetic variance, including additive, dominance, and genic interaction variances. Interpretation: A high H² suggests that a large portion of the phenotypic variation is due to genetic factors, including non-additive effects. Definition : Narrow-sense heritability focuses specifically on the additive genetic variance, representing the proportion of phenotypic variance passed from parents to offspring due to additive genetic factors. Interpretation : Narrow-sense heritability focuses specifically on the additive genetic variance, representing the proportion of phenotypic variance passed from parents to offspring due to additive genetic factors.
- 17. IN SUMMARY, BROAD-SENSE HERITABILITY CONSIDERS A BROADER RANGE OF GENETIC INFLUENCES, WHILE NARROW-SENSE HERITABILITY FOCUSES SPECIFICALLY ON ADDITIVE GENETIC EFFECTS. BOTH MEASURES HELP IN UNDERSTANDING THE GENETIC BASIS OF TRAITS WITHIN POPULATIONS, GUIDING DECISIONS IN FIELDS SUCH AS AGRICULTURE, ANIMAL BREEDING, AND HUMAN GENETICS.
- 19. MEASUREMENT OF HERITABILITY 1. By Elimination of variance components •First method of calculating broad-sense heritability includes the elimination of one of the components of phenotypic variance. •We have, VP = VG + VE + VGE •If we eliminate all environmental variance (VE = 0), then VGE = 0 and VP = VG. But in practice, it is virtually impossible to ensure that all individuals are raised in exactly the same environment. •Instead, we could make VG = 0 by raising genetically identical individuals, causing VP = VE. •In a typical experiment, we might raise cloned or highly inbred, identically homozygous individuals in a defined environment and measure their phenotypic variance to estimate VE. We might then raise a group of genetically variable individuals and measure their phenotypic variance (VP).
- 20. Thus, we could obtain: VG(of genetically varying individuals) = VP(of genetically varying individuals) − VE (of genetically identical individuals) This method assumes that the environmental variance of genetically identical individuals is the same as the environmental variance of genetically variable individuals, which may not be true. Moreover, this approach can be applied only to organisms for which it is possible to create genetically identical individuals.
- 21. EXAMPLE- SEWALL WRIGHT ESTIMATED THE HERITABILITY OF WHITE SPOTTING IN GUINEA PIGS. First the phenotypic variance of white spotting in a genetically variable population, VP = 573. Then, the guinea pigs were for many generations so that they were homozygous and genetically identical. Then the phenotypic variance, VP = 340. Because VG = 0 in this group, their VP = VE. • Estimate their genetic variance: VP − VE = VG 573 − 340 = 233 • Estimating broad- sense heritability: H2 = VG/VP H2 = 233/573 = 0.41
- 22. •Another method of estimating heritability is to compare the phenotypes of parents with those of their offspring. •To calculate the narrow-sense heritability of a characteristic in this way, we first measure the characteristic in a series of parents and offspring. The data are arranged into families, and the mean parental phenotype is plotted against the mean offspring phenotype. Assuming that there is no narrow-sense heritability for the characteristic (h2 = 0), meaning that genetic differences do not contribute to the phenotypic differences among individuals. •In this case, offspring will be no more similar to their parents than they are to unrelated individuals, and the data points will be scattered randomly, generating a regression coefficient of 0. •Now, let us assume that all of the phenotypic differences are due to additive genetic differences (h2 = 1). In this case, the mean phenotype of the offspring will be equal to the mean phenotype of the parents, and the regression coefficient will 2. BY PARENT-OFFSPRING REGRESSION
- 23. •Finally, if genes and environment both contribute to the phenotypic differences, both heritability and the regression coefficient will lie between 0 and 1. Therefore, the regression coefficient provides information about the magnitude of heritability. •A complex mathematical proof demonstrates that: h2 (narrow-sense heritability) = b(regression of offspring mean against mean of both parents) •Sometimes, only the phenotype of one parent is known, then: h2 = 2b(regression of offspring mean against mean of one parent) TO BE CONTINUED..
- 24. •A third method for calculating heritability is to compare the phenotypes of individuals with different degrees of relatedness. •The concept says that the more closely related two individuals are, the more genes they share. •Monozygotic (identical) twins share 100% of their genes, whereas dizygotic (non- identical) twins share, on average, 50% of their genes. •If genes are important in determining variation in a characteristic, then monozygotic twins should be more similar in that characteristic than dizygotic twins. 3. BY DEGREE OF RELATEDNESS
- 25. •A rough estimate of broad-sense heritability can be obtained by: H2 = 2(rMZ − rDZ) •Where rMZ equals the correlation coefficient among monozygotic twins and rDZ equals the correlation coefficient among dizygotic twins. •For example, suppose we found the correlation of height among the two members of monozygotic twin pairs (rMZ) to be 0.9 and the correlation of height among the two members of dizygotic twin pairs (rDZ) to be 0.5. •The broad-sense heritability for height would be H2 = 2(0.9 − 0.5) = 2(0.4) = 0.8. •This calculation assumes that the two members of a monozygotic twin pair experience environments that are no more similar to each other than those experienced by the two members of a dizygotic twin pair. This assumption is often not met when twins have been reared together. . TO BE CONTINUED..
- 26. 1.Heritability does not indicate the degree which a characteristic is genetically determined. 2.An individual does not have heritability 3.There is no universal heritability for a characteristic 4.Even when heritability is high, environmental factors can influence a characteristic 5.Heritabilities indicate nothing about differences among populations LIMITATIONS OF HERITABILITY
- 27. QTL MAPPING (QUANTITATIVE TRAIT LOCI) RESPONSE TO SELECTION
- 28. QTL MAPPING (QUANTITATIVE TRAIT LOCI) •Quantitative trait loci (QTL) are genetic regions that influence phenotypic variation of a complex trait, often through genetic interactions with each other and the environment. These are commonly identified through a statistical genetic analysis known as QTL mapping. (Powder, 2020). • Quantitative trait locus (QTL) analysis is a statistical method that links two types of information—1. phenotypic data (trait measurements) and 2. genotypic data (usually molecular markers)—in an attempt to explain the genetic basis of variation in complex traits. (Falconer & Mackay, 1996; Kearsey, 1998; Lynch & Walsh, 1998). agriculture, evolution, and medicine • QTL analysis allows researchers in fields as diverse as to link certain complex phenotypes to specific regions of chromosomes. The goal of this process is to identify the action, interaction, number, and precise location of these regions.
- 29. WHY QTL ANALYSIS ? •Quantitative trait loci (QTL) is a locus (section of DNA) which corelates with variation of a quantiatative trait in the phenotype of a popualation of an organisms. QTL’s are mapped by ide3ntifying which molecular marker (such as SNPs or AFLPs) correlated with an observed trait. • It is helpful in assessing a possible number of loci, their distribution in the genome, equality of effects, and manner of their action. DNA markers are very useful for information about numbers and position of QTL’s because they are highly polymorphic, abundant and co-dominant in nature. Since a single locus may include many variants, imputation or whole-genome sequencing is a key prerequisite for QTL mapping to enable precise identification of the contributing molecular marker. QTLs have been expanded to include variants that act at different levels throughout the genotype-to- phenotype continuum. Fig: Flow of information from collecting genotypic and phenotypic data to mapping to graphical genotyping and quantitative trait locus analysis. Figure credit: Heather Merk. Image credits: Genotyping, Allen Van Deynze, UC Davis; Mapping, Scott Wolfe, The Ohio State University
- 30. TYPES OF QTL ANALYSIS QTL analysis is an effective means of annotating variants that are associated with disease. By understanding the functional effects of variants, it allows for the distinction between variants that are involved with disease, from those that are correlated with disease. By leveraging different QTL analyses, the network of molecular interactions of variants and the genes they affect begin to come into view, and provide evidence for which underlying genes and pathways are truly driving disease. This enables the investment of time, resources, and funding in targets that are most likely to be involved with disease.
- 32. SOFTWARES FOR QTL MAPPING Step 1: search Genenetwork software in chrome Step 2: Visit the website and fill data as instructed Step 3: Observe the
- 33. INTENSITY OF SELECTION (I)
- 35. ACCURACY & LIMIT OF SELECTION
- 38. REFERENCE PAPERS & SITES: Haworth, C. M., & Plomin, R. (2010). Quantitative genetics in the era of molecular genetics: learning abilities and disabilities as an example. Journal of the American Academy of Child and Adolescent Psychiatry, 49(8), 783–793. https://doi.org/10.1016/j.jaac.2010.01.026 Khoury, M. J., Janssens, A. C., & Ransohoff, D. F . (2013). How can polygenic inheritance be used in population screening for common diseases?. Genetics in medicine : official journal of the American College of Medical Genetics, 15(6), 437–443. https://doi.org/10.1038/gim.2012.182 Shinozuka, H., Cogan, N. O., Spangenberg, G. C., & Forster, J. W. (2012). Quantitative Trait Locus (QTL) meta-analysis and comparative genomics for candidate gene prediction in perennial ryegrass (Lolium perenne L.). BMC Genetics, 13, 101. https://doi.org/10.1186/1471-2156-13-101 Tuberosa, R., Salvi, S., Sanguineti, M. C., Landi, P., Maccaferri, M., & Conti, S. (2002). Mapping QTLs regulating morpho-physiological traits and yield: case studies, shortcomings and perspectives in drought-stressed maize. Annals of botany, 89 Spec No(7), 941–963. https://doi.org/10.1093/aob/mcf134 https://www.thoughtco.com/genetic-variation-373457