Genetics of Quantitative Traits

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Genetics of Quantitative Traits
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Quantitative Trait

• Any trait that demonstrates a range of phenotypes
  that can be quantified
• Height
• Weight
• Coloration
• Size

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Continuous Variation vs Discrete Phenotypic
Classes

• Continuous variation
• Offspring show a range of phenotypes of
  intermediate range relative to the parental
  phenotype extremes
• Discrete classes
• Offspring show phenotype exactly like either
  parent (dominance/recessiveness)
• or in a single intermediate class (incomplete
  dominance)
• or have a combinatorial phenotype (co-dominance)

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Example of Continuous Variation
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Demonstrating Genetic Control of Variation

• Individually cross F2 at phenotypic extremes
• Subsequent ranges of progeny are centered on F2
  phenotype

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Polygenic Inheritance

• A trait controlled by multiple genes with
  additive and non-additive allele types
• Additive allele (Uppercase)
• an allele which contributes to the observe
  phenotype
• causes more color, height, weight, etc..
• Non-additive allele (lowercase)
• an allele which does not contribute to observed
  phenotype
• causes less color, height, weight, etc

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Polygenic Control of Wheat Color
P
F1
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Wheat Color Defined by Two Genes

• A and B are additive alleles of two genes
• a and b are non-additive alleles of the same two
  genes
• The number of additive and non-additive alleles
  in each genotype defines a distinct phenotype
• 4 additive alleles ------? AABB
• 3 additive alleles ------? AaBB, AABb,
• 2 additive alleles ------? aaBB, AAbb, AaBb
• 1 additive allele -------? Aabb, aaBb
• 0 additive alleles ------? aabb
• Give 5 phenotype classes

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How Many Genes Control a Trait? How Many
Phenotypes are Possible?
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Statistics
Numbers of individuals with that phenotype
Range of the phenotype being measured
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Mean (aka Average) and Variance
Height of Population 2
Number of Individuals with Indicated Height
Height of Population 1
1ft
2.5ft
10ft
7.5ft
(Height)

• These two populations have a mean height that is
  the same
• The range of heights in each population is quite
  different

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Measuring the Variance

• Sample variance s2
• Standard deviation square root of variance
• Standard error

n of individuals for which trait has been
quantified
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Weight Distribution of F1 F2 Tomato Progeny
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Example Statistics Problem
12.04 1.13
12.11 2.06
See table 6.4 (4th ed) or table 5.4 (3rd ed)
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Nature or Nurture

• Phenotypic variation due to genetic factors
• Phenotypic variation due to environmental factors
• Heritability
• Broad-sense
• Measure of variance due to genetics vs
  environment
• Narrow-sense
• Measure of selectability

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Identifying Environmental vs Genetic Factors
Influencing Variability

• Inbred strains
• an inbred population is highly homozygous
• lethal recessives are lost
• allele frequencies are stabilized
• Variation in inbred populations in differing
  environments is due to environmental factors VE
• Variation in inbred population in same
  environment is due to genetic differences - VG

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Environmental vs Genetic Factor Measurement

• If extreme phenotypes of highly inbred line are
  selected, do F1 show deviation from P mean?
• yes variance is genetic
• no variance is environmental

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Broad-sense Heritability

• Heritability index H2
• Proportion of variance due to genetic factors
• VP phenotypic variance (ie s2 for a measured
  trait in a population)
• VP VE VG
• VG genetic variance
• VE environmental variance

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Narrow-sense Heritability

• S deviation of selected population mean from
  whole population mean
• R deviation of offspring mean from whole
  parental population mean
• ratio of R to S describes narrow-sense
  heritability ie how selectable is the trait

h2 near 1 means trait could be altered by
artificial selection
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