Quantitative Inheritance

• Continuous trait: continuous gradation from one phenotype to the next (example: growth rate, crop yield, weight)
• Meristic trait: phenotype falls into discrete, integral categories; discontinuous (example: ability to roll tongue, litter size in cats)
• Threshold trait: only 2 or a few phenotypic classes, but its inheritance is determined by multiple genes and environment; expression implies affected individuals has liability over the threshold (example: diabetes, schizophrenia, certain cancers)
• Gaussian distribution or bell curve: these curves are characterized by the mean (mid-point) and by the variance (width).
• Standard deviation, the square root of variance, is used as a measure of the curve’s width.
• Mendelian traits are discontinuous traits.
• 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.
• Liability: the factors which influence the development of a polygenic disorder, whether genetic or environmental.
• The liabilities of all individuals in a population form a discontinuous variation, and therefore cannot be plotted in a Gaussian fashion.
• (threshold model) 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.
• 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.
• Polygenic trait: 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.
• Polygene is also called multiple factor or quantitative gene.
• 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.
• Not all human traits that exhibit normal distribution are polygenic.
• Because segregating populations exhibit a continuous distribution for quantitative traits, they cannot be analyzed using traditional Mendelian genetic techniques.
• Because segregating populations exhibit a continuous distribution for quantitative traits, they are described by statistical parameters.
• Genetic Sources of Variation

  1. additive variance (VA)
  2. dominance variance (VD)
  3. epistatic variance (VI)
• The sum of these subcategories of genetic sources of variation yield the total amount of genetic variation (VG) responsible for a particular phenotypic trait.
• Additive variation (VA) represents the cumulative effect of individual loci, therefore the overall mean is equal to the summed contribution of these loci.
• Dominance variation (VD) 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.
• All instances of phenotypic variance (VP) within a population are the result of genetic sources (VG) and/or environmental sources (VE).
• The total phenotypic variation of a population (VP) is the sum of the variation in additive (A), dominance (D), gene-interaction (I), environmental (E) and gene-environment interaction (GE) effects.
• Variation in additive (VA): Expression of a trait IS NOT controlled by the other allele at the locus.
• Variation in Dominance (VD): Expression of a trait IS affected by the other allele at the locus
• Variation in Gene-Interaction (VI ): Expression of a trait is affected by alleles at another loci.
• Variation in Gene-Environment Interaction (VGE): A given genotype is superior to another in one environment (differential local adaptation).
• Heritability

  • how much of the phenotypic variability of a trait is due to genetic variance (estimate but not absolute)
  • how much is due to non-genetic environmental factors
• Broad-sense heritability: h^2 = VG/VP
• Narrow-sense heritability: h^2 = VA/VP
• h^2 is the proportion of variability that can be passed on from parent to offspring.
• h^2 = 0 means that:

  • none of the phenotypic variance among individuals is due to additive genetic differences (VA=0)
  • offspring will NOT closely resemble their parents for the trait of interest for genetic reasons
• h^2 = 1 means that

  1. all the variation among individuals is due to heritable genetic differences (VP=VA )
  2. offspring will resemble their parents very closely