The document discusses quantitative inheritance, covering the inheritance of quantitative traits influenced by multiple genes, including continuous, meristic, and threshold traits. It explores concepts such as heritability, components of phenotypic variance, and the role of environmental factors in expressing these traits. Regression to the mean and the importance of understanding genetic versus non-genetic sources of variation are also emphasized.

1. BIO 106
Lecture 10
Quantitative Inheritance

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
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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
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4. Continuous traits
Quantitative Inheritance
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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
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6. Meristic traits
Quantitative Inheritance
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7. Mendelian traits are discontinuous traits.
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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.
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9. Threshold traits
Quantitative Inheritance
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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.
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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.
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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.
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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
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14. Quantitative Inheritance
Regression to the Mean (by Francis Galton)
Most offspring of extreme parents are more
average than their parents.
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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?
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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.
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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
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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.
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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
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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
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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
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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
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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
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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.
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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.
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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
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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
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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.
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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.
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30. 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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31. Quantitative Inheritance
Examples of
heritable
traits
myopia
Mass of the brain
Hip dysplasia in dogs
dimples
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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.
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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
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