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Genetics: Quantitative Inheritance


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BIO 106
Lecture 10

Quantitative Inheritance
A. Inheritance of Quantitative Characters
1. Multiple Genes
2. Number of Genes in polygene Systems
3. Regression to the Mean
4. Effects of Dominance and Gene Interactions
5. Effects of Genes in Multiplying Effects
B. Analysis of Quantitative Characteristics
C. Components of Phenotypic Variance
D. Heredity
1. Heritability in the Narrow Sense
2. Heritability in the Broad Sense

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Genetics: Quantitative Inheritance

  1. 1. BIO 106 Lecture 10 Quantitative Inheritance
  2. 2. Quantitative Inheritance A. Inheritance of Quantitative Characters 1. Multiple Genes 2. Number of Genes in polygene Systems 3. Regression to the Mean 4. Effects of Dominance and Gene Interactions 5. Effects of Genes in Multiplying Effects B. Analysis of Quantitative Characteristics C. Components of Phenotypic Variance D. Heredity 1. Heritability in the Narrow Sense 2. Heritability in the Broad Sense 2ccex2015
  3. 3. QuantitativeInheritanceTypes of Quantitative Trait: 1. continuous trait - continuous gradation from one phenotype to the next; continuum of phenotypes example: growth rate, crop yield, weight 2. meristic trait - phenotype falls into discrete, integral categories; discontinuous example: ability to roll tongue, litter size in cats 3. threshold trait - only 2 or a few phenotypic classes, but their inheritance is determined by multiple genes and environment - expression implies affected individuals has liability over the threshold example: diabetes, schizophrenia, certain cancers 3ccex2015
  4. 4. Continuous traits Quantitative Inheritance 4ccex2015
  5. 5. Continuous traits do not fall into discrete categories. In general, their values in a population follow the NORMAL DISTRIBUTION (also known as Gaussian distribution or bell curve). These curves are characterized by the mean (mid-point) and by the variance (width). Often standard deviation, the square root of variance, is used as a measure of the curve’s width. Milk yield 5ccex2015
  6. 6. Meristic traits Quantitative Inheritance 6ccex2015
  7. 7. Mendelian traits are discontinuous traits. 7ccex2015
  8. 8. A threshold trait has an underlying quantitative distribution, but the trait appears only if a threshold is crossed. Only those individuals exceeding the threshold on the liability scale will express the trait. 8ccex2015
  9. 9. Threshold traits Quantitative Inheritance 9ccex2015
  10. 10. All of the factors which influence the development of a polygenic disorder, whether genetic or environmental, can be considered as a single entity known as LIABILITY. The liabilities of all individuals in a population form a discontinuous variation, and therefore CANNOT be plotted in a Gaussian fashion, However, the curves for these relatives will be shifted to the right, with the extent to which they are shifted being directly related to the closeness of their relationship to the affected index case. Liability / threshold model Liability is not directly observable. 10ccex2015
  11. 11. Quantitative Inheritance Consequences of Liability Model: • The incidence of the condition is greatest among relatives of the most severely affected patients. • The risk is greatest among close relatives and decreases rapidly in more distant relatives. • If there is more than one affected close relative then the risks for other relatives are increased. 11ccex2015
  12. 12. Quantitative Inheritance Multiple genes polygenic trait is one whose phenotype is influenced by more than one gene Polygene – any group of non-allelic genes, each having a small quantitative effect, that together produce a wide range of phenotypic variation; - also called multiple factor, quantitative gene. 12ccex2015
  13. 13. Quantitative Inheritance There are several but not an unlimited number of genes involved in the expression of a polygenic trait. The loci act in concert in an additive fashion. The phenotype is a result of the interaction of the genotype and the environment. But not all human traits that exhibit normal distribution are polygenic. Number of Genes in Polygene Systems 13ccex2015
  14. 14. Quantitative Inheritance Regression to the Mean (by Francis Galton) Most offspring of extreme parents are more average than their parents. 14ccex2015
  15. 15. Quantitative Inheritance Ex: PQRST = tallness genes PpQQRRsstt x PPQqRRSstt = PPQqRRSstt PPQqRRSsTt x ppQqRrSsTt = PpQqRrSsTt Suppose PPQQRrSsTt X PpQqRRSsTT = PPQqRrSsTT Relative to his parents, how tall is this offspring? The product of which of the above crosses matches the info in the graph? 15 ccex2015
  16. 16. Quantitative Inheritance Statistical Analysis of Quantitative Traits Because segregating populations exhibit a continuous distribution for quantitative traits, they cannot be analyzed using traditional Mendelian genetic techniques. They are described by statistical parameters. 16ccex2015
  17. 17. Quantitative Inheritance Genetic Sources of Variation • can themselves be divided into several subcategories, including additive variance (VA), dominance variance (VD ), and epistatic variance (VI). Together, the values for each of these subcategories yield the total amount of genetic variation (VG) responsible for a particular phenotypic trait: • VG = VA + VD + VI 17ccex2015
  18. 18. Quantitative Inheritance Additive variation represents the cumulative effect of individual loci, therefore the overall mean is equal to the summed contribution of these loci. Dominance variation represents interaction between alleles. If a trait is controlled by a dominant allele, then both homozygous and heterozygous individuals will display the same phenotypic value. 18ccex2015
  19. 19. Quantitative Inheritance Components of Phenotypic Variance All instances of phenotypic variance (VP) within a population are the result of genetic sources (VG) and/or environmental sources (VE). VP = VG + VE 19ccex2015
  20. 20. Quantitative Inheritance Components of Phenotypic Variance The total phenotypic variation (V) of a population is the sum of the variation in additive (A), dominance (D), gene- interaction (I), environmental (E) and gene-environment interaction (GE) effects: VP = VA + VD + VI + VE + VGE 20ccex2015
  21. 21. Components of Phenotypic Variance VP = VA + VD + VI + VE + VGE Meaning: VA = Expression of a trait IS NOT affected by the other allele at the locus. VD = Expression of a trait IS affected by the other allele at the locus. VI = Expression of a trait is affected by alleles at another loci. VGE = A given genotype is superior to another in one environment (differential local adaptation). Quantitative Inheritance 21ccex2015
  22. 22. Quantitative Inheritance Heritability Plant and animal breeders should know • how much of the phenotypic variability of a trait is due to genetic variance • how much is due to non-genetic environmental factors. broad-sense heritability: h2 = VG/VP 22ccex2015
  23. 23. Quantitative Inheritance Heritability It is even more useful to know what proportion of the phenotypic variation is due to additive genetic effects. The heritability (narrow-sense) of a trait is the proportion of the total phenotypic variation that is due to heritable (additive genetic) effects: h2 = VA/VP 23ccex2015
  24. 24. Quantitative Inheritance Heritability h2 is the proportion of variability that can be passed on from parent to offspring. h2 = 0 means that none of the phenotypic variance among individuals is due to additive genetic differences (VA=0) SO offspring will NOT closely resemble their parents for the trait of interest for genetic reasons. 24ccex2015
  25. 25. Quantitative Inheritance Heritability • When h2 = 1, all the variation among individuals is due to heritable genetic differences (VP=VA) and offspring will resemble their parents very closely. 25ccex2015
  26. 26. Some HERITABILITY facts: Its value can range from 0 (no genetic contribution) to 1 (all differences on a trait reflect genetic variation). It does NOT apply to individuals but to populations (reflects the fact that all individuals in any species of living things differ in many ways among each other.) Quantitative Inheritance 26ccex2015
  27. 27. Some HERITABILITY facts: A heritable trait is not necessarily adaptive. Every gene must express itself in an environment, and all environments must act on the genotype an individual gets. Quantitative Inheritance 27ccex2015
  28. 28. Quantitative Inheritance Some HERITABILITY facts: Finding no heritability for the trait is not a demonstration that genes are irrelevant; rather, it demonstrates that, in the particular population studied, there is no genetic variation at the relevant loci or that the environments in which the population developed were such that different genotypes had the same phenotype. 28ccex2015
  29. 29. Quantitative Inheritance Some HERITABILITY facts: A high heritability does not mean that a trait is unaffected by its environment. In general, the heritability of a trait is different in each population and in each set of environments; it cannot be extrapolated from one population and set of environments to another. 29ccex2015
  30. 30. Quantitative Inheritance language you speak which particular church you worship Examples of non-heritable traits trait that results from physical damage 30ccex2015
  31. 31. Quantitative Inheritance Examples of heritable traits myopia Mass of the brain Hip dysplasia in dogs dimples 31ccex2015
  32. 32. Quantitative Inheritance Twin studies Theoretically, any phenotypic differences between identical twins are environmental. Phenotypic differences between fraternal twins can be due to both environmental and genetic differences. 32ccex2015
  33. 33. Quantitative Inheritance If the heritability is high, • identical twins will normally be very similar for a trait • fraternal twins will be less similar If the heritability is low, • identical twins may not be much more similar than fraternal twins. If variation for a trait is completely heritable, • identical twins should be have a correlation near 1 • fraternal twins should have a correlation near 0.5 33ccex2015