Download - Genetics: Quantitative Inheritance
BIO 106
Lecture 10
Quantitative Inheritance
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 2 ccex2015
Quantit
ativ
e In
her
itance Types 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
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Continuous traits
Quantitative Inheritance
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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
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Meristic traits
Quantitative Inheritance
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Mendelian traits are discontinuous traits.
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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.
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Threshold traits
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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.
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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.
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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.
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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
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Quantitative Inheritance
Regression to the Mean (by Francis Galton)
Most offspring of extreme parents are more average than their parents.
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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?
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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.
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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
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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.
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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
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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
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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
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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
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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
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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.
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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.
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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
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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
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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.
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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.
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Quantitative Inheritance
language you speak
which particular church you worship
Examples of non-heritable
traits
trait that results from physical damage
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Quantitative Inheritance
Examples of heritable
traits
myopia
Mass of the brain
Hip dysplasia in dogs
dimples
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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.
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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
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