Download Quantitative Genetics and Multifactorial Traits

Quantitative Genetics and Multifactorial Traits
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The genetic analysis of complex characteristics is quantitative genetics
Complex characteristics usually are either polygenic or are influenced by
environmental factors
Characteristics that are both polygenic and influenced by environmental
factors are multifactorial
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Sometimes the relationship between genotype and phenotype is fairly easy
to understand
o In some instances, it is essentially additive
o In the case of quantitative characteristics, the phenotypes are not
distinct and can be quite hard to interpret
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But not all quantitative characteristics exhibit continuous phenotypic
variation - simply because they can’t
o Litter size has to yield whole numbers but the underlying genetics can
exhibit quantitative influences (polygenic and environmental factors)
o These are referred to as meristic characteristics
Threshold characteristics can also be confusing
o There are only two phenotypes exhibited but the underlying genetics
exhibit quantitative influences
o In this case, susceptibility is determined by a number of factors
o When the susceptibility (liability or risk) is larger than the threshold
level, the disease is seen
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Inheritance for a polygenic characteristic can be broken up into the
inheritance for each individual gene
The trick is that after you break the crosses apart, you need to put the results
back together
As the number of loci affecting a phenotype increases, the number of
phenotypic classes increases even more
Under some circumstances, we can estimate the number of genes involved
o If we can begin with two parents who are homozygous for different
alleles at each locus, the F1 will all be heterozygous
o If we mate two F1, then (1/4)n will be the number of F2 individuals
that should resemble each of the original homozygous parents
Statistical Analysis
We can use some basic statistical tools to analyze the genetics behind quantitative
characteristics
 There are three common types of distribution curves
 Samples and populations
o When sampling is completely random and large, it can provide
information about the entire population
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Variance provides information on how spread out a distribution is
o The greater the variance, the more spread out the distribution is
Standard deviation
o We can expect various percentages of the samples to fall within the
ranges determined by the standard deviation
Correlation refers to a relationship between two related characteristics
o Correlation coefficient (r)
o Covariance
 Correlation coefficients can range from -1 to +1
 Coefficients of -1 or +1 indicate perfect correlations
 Coefficients near these values indicate strong correlations
 Coefficients near 0 indicate weak or no correlation
Heritability
o The proportion of phenotypic variation due strictly to genetics
o Phenotypic variance
o VP = VGenetic + VEnvironmental + VGE
o Genetic variance can be further subdivided
o VG = VA + VD + VI
o VA = additive genetic variance
o VD = dominance genetic variance
o VI = genic interaction variance
o Therefore, VP = (VA + VD + VI)+ VE + VGE
o Broad-sense heritability (H2)
o Can range from 0-1
o 0 = no genetic influence, all environmental
o 1 = total genetic influence, no environmental
o We can also consider how much is due to additive genetic variance
o Narrow-sense heritability (h2)
o This is especially useful to breeders because it determines how much
the offspring will resemble the parents
o Calculating heritability
o Elimination of variance components
 Since VP = VG + VE + VGE, if we set VG=0, then VP = VE
 In practice, we might run two sets of experiments
1. One with genetically-identical individuals - This would give
us VE (gen identical)
2. One with genetically-variable individuals - This would give
us VP (gen variable)
o VG (gen variable) = VP (gen variable) - VE (gen identical)
For example, if VP (gen variable) = 573, and VP (gen identical) = 340 when
VG (gen identical) = 0
o Then VP (gen identical) = VE (gen identical)
o VP - VE = VG so 573 - 340 = 233
Heritability tells us how much of the phenotypic variance is due to genetic variance
o Indicates the degree to which genes determine variation of a characteristic in
the test group
o It does not tell us the degree to which a characteristic is genetically
determined
o Heritability deals only with groups of individuals, not the individuals by
themselves
o The particular group is also important
o Since heritability is unique for a particular population, it cannot be used to
determine the reason for phenotypic differences between populations
Response to selection
o Artificial selection has been practiced by humans for centuries
o The amount a characteristic subjected to selective pressure changes in one
generation
o response to selection = mean phenotype of the offspring - mean phenotype of
the original population
o Depends on
1. Narrow-sense heritability
How much do the offspring resemble the parents?
2. Selection differential
o Difference between the mean phenotype of the selected parents and the
mean phenotype of the original population=
R = h2 x S
o R = response to selection; h2 = narrow-sense heritability; S = selection
differential
o R predicts the expected change in the offspring relative to the original
population
o Heritability determined by a response-to-selection experiment is
termed the realized heritability
o Eventually the response to selection will level off
o We can also use individuals with different degrees of relatedness
o Can compare results for monozygotic (MZ) twins vs. dizygotic (DZ)
o H2 = 2(rMZ - rDZ)
o Chromosomal regions that contain genes influencing quantitative traits
are called quantitative trait loci (QTLs)
o Mapping QTLs
o This is a complicated task with many different factors to consider
o Correlated responses
o Phenotypic correlation is seen when two phenotypes are
correlated
o This can be due to environmental or genetic correlations
o We can see examples of other correlated phenotypes in the
domestication of dogs