Genetics: Quantitative Inheritance

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 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


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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


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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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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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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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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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Regression to the Mean (by Francis Galton)

Most offspring of extreme parents are more average than their parents.

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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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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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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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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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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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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).


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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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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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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.)


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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.


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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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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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language you speak

which particular church you worship

Examples of non-heritable

traits

trait that results from physical damage

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Examples of heritable

traits

myopia

Mass of the brain

Hip dysplasia in dogs

dimples

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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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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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Education

Genetics: Quantitative Inheritance