GEN-3000 Lecture Notes - Lecture 20: Soil Fertility, Coronary Artery Disease, Locule

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7 Dec 2020
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Chapter 20 - Quantitative Genetics and Multifactorial Traits
Introduction
Many traits not distinct and clear-cut show a continuous range of
phenotypes:
Height in humans, milk and meat production in cattle, and yield in
various crops
Continuous variation:
Measured and described in quantitative terms known as
quantitative inheritance
Quantitative inheritancegenetic phenomenon:
Continuous variation can be measured in quantitative terms.
Polygenic (many genes):
Varying phenotypes result from input of genes at more than one
loci.
Genes involved referred to as polygenes
Multifactorialcomplex traits:
Quantitative (polygenic) traits
Phenotypes results from both gene action and environmental
influences.
Multiple-Gene Hypothesis
Quantitative patterns of inheritance:
Mendelian factors could not account for range of phenotypes
seen in quantitative patterns of inheritance.
Multiple-gene hypothesis (multiple-factor):
Many genes, individually behaving in Mendelian fashion,
contribute to phenotype in a cumulative/quantitative way.
Example: grain color in wheatFigure 20-1
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Figure 20-1
How the multiple-factor hypothesis accounts for the 1:4:6:4:1 phenotypic ratio of grain color when all
alleles designated by an uppercase letter are additive and contribute an equal amount of pigment to
the phenotype.
Additive and Non-additive Alleles
Multiple-gene hypothesisadditive alleles and nonadditive alleles:
Various grain color phenotypes due to additive and nonadditive
alleles
Additive allele: contributes equally to red grain color
Nonadditive allele: fails to produce red pigment
Greater number of additive alleles in genotype = more intense red color
expressed in phenotype.
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Quantitative Loci
Quantitative loci:
Numerous loci can function in similar fashion.
Greater and greater numbers of classes appear in F2 generation
in more complex ratios.
Figure 20-2
Figure 20-2
The genetic ratios (on the x-axis) resulting from crossing two heterozygotes when polygenic
inheritance is in operation with 1 to 5 gene pairs. The histogram bars indicate the distinct F2
phenotypic classes, ranging from one extreme (left end) to the other extreme (right end). Each
phenotype results from a different number of additive alleles.
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