Classical Mendelian Traits

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Classical Mendelian Traits
What are some of the traits, or characters that
we have considered up to now?
What different phenotypes did we observe?
How would you describe these phenotypes, discrete
or continuous?
Generally, how many genes controlled these traits?
How are these traits usually studied?
How are the data analyzed?
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• Quantitative Genetics
• So far we have only considered discrete traits
  (tall or dwarf, red or white, wrinkled or
  smooth)
• Sometimes you dont get discrete categories of
  offspring, but a range of traits that are not
  easily categorized into classes
• This is referred to as continuous variation

How can a continuous trait be heritable ?
How can a heritable trait be continuous ?
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The Big Debate
Who was correct?
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Ronald A. Fischer (1890-1962)
His statistical analysis of the inheritance of
complex traits established a functional link
between Mendels discoveries and Darwins ideas
on natural selection
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• Whats behind continuous variation?
• Bateson and Yule suggested that it could be
  explained by a large number of factors including
  genes and environment
• Multiple-factor or multiple-gene hypothesis
• Each factor contributes additively or
  cumulatively to the trait

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Seed Weight
What ratio do you expect for the F2 generation?
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Seed Weight
Can we draw on our previous knowledge to help us
explain this?
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Recall the Case of Incomplete Dominance
But this doesnt completely explain it.
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Lets Consider Two Genes With Incomplete
Dominance
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Continous Variation Is Seen in Wheat Color
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Continous Variation Is Seen in Wheat Color
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• Wheat Grain Color
• Crossing true-breeding red with true-breeding
  white gives F1 that is intermediate in phenotype
• The F2 offspring are 15/16 some shade of red,
  1/16 white ( of genes?)
• Still follows Mendels rules
• Can define four discrete phenotypic shades of
  red
• Each gene has one additive and one non-additive
  allele
• Write additive alleles as capitals (A, B) and
  non-additive as lowercase (a,b)

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• Conclusions
• P1 both homozygous, one for only additive
  alleles (AABB), one for only non-additive (aabb)
• F1 heterozygous (AaBb) with two additive (AB)
  and two non-additive (ab) alleles
• F2 has either 4, 3, 2, 1 or 0 additive alleles
  
• 1/16 has 4, 1/16 has 0 (resemble the
  P1)
• 3, 2, 1 make up other red categories
• 2 is most frequent (resembles F1)
• 14641

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• Continuous Variation is Mendelian
• Two genes gave a 14641 ratio
• 16 genotypes with five phenotypic classes
• There is no reason why more genes couldnt act
  this way

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Continous Variation Is Seen in Wheat Color
64
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Increasing the Number of Genes Increases the
Phenotypes
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• Calculating Numbers of Continuous Genes Another
  Way
• The number of genes determines the number of
  distinct phenotypes in the F2 generation
• Relationship is Number of F2 phenotypes (2n
  1) where n number of genes
• If n2, (2n1) 5 (wheat color example)
• If n5, (2n1) 11

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• Calculating Numbers of Continuous Genes
• If you can figure out the ratio of F2s that
  resemble the P1s you can calculate the number of
  genes involved (n) using
• 1/4n ratio of F2 with either P1 phenotype
• - Wheat example 1/16 dark red or 1/16 white
  
• i.e. 1/4n 1/16 4n 16 ... 42 16
• Therefore n 2 genes

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Comparing Qualitative and Quantitative Genetics
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• Quantitative Genetics
• So far we have only considered discrete traits
  (tall or dwarf, red or white, wrinkled or
  smooth)
• Sometimes you dont get discrete categories of
  offspring, but a range of traits that are not
  easily categorized into classes
• This is referred to as continuous variation
• Often controlled by two or more genes that
  provide an additive component (polygenic
  inheritance)
• Must consider large groups of organisms as
  opposed to individuals
• Analyze average values and variation within and
  between groups of organisms

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• Additive Alleles
• Characters can be quantified (e.g. weighing)
• Two or more gene pairs throughout the genome
  control the trait
• Each allele can be additive or non-additive
• All the alleles contribute a small, equal
  amount to the character and together produce
  substantial variation
• Large numbers of offspring are needed

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Recommended problems for Chapter 6 Pages 117
118 1, 4, 5, 8, 10, 12, 13
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Normal Distribution Curves
What does a frequency distribution tell us?
What does normal distribution mean?
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Frequency Distribution for Height
Average height for this group is 571/3
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Comparing Quantitative Phenotypes
A
B
The first step is to determine the mean value of
population A and the mean value of population B.
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Statistical Analysis
Mean
1, 3, 5, 7
2, 5, 5, 4
135716
16/44
16/44
255416
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Comparing Quantitative Phenotypes
A
B
If the mean value of population A is equal to the
mean value of population B, are these two
populations equivalent?
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Statistical Analysis
Mean
Variance
Std. Deviation (SD)
Std. Error of the Mean (SEM)
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Frequency Distribution for Height
Average height for this group is 571/3
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• Genotype is Not the Only Factor Effecting
  Phenotype
• Environment
• - temperature sensitive or conditional
  expression
• Nutritional effects
• - enriched or deprived
• Treatments with hormones or drugs
• - create phenocopies

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• Quantitative Genetics
• So far we have only considered discrete traits
  (tall or dwarf, red or white, wrinkled or
  smooth)
• Sometimes you dont get discrete categories of
  offspring, but a range of traits that are not
  easily categorized into classes
• This is referred to as continuous variation

How can a continuous trait be heritable ?
How can a heritable trait be continuous ?
How heritable is a continuous trait?
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Phenotypic Variance
Phenotypic variance (VP) is due to the sum of
three factors

• Environmental variance (VE)
• Genetic variance (VG)
• Genetic/Environmental Interaction variance (VGE)

VP VE VG VGE
VP VE VG
H2 VG / VP
Heritability in the broad sense is an estimate of
the proportion of observed phenotypic variation
due to genetic factors.
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Heritability Estimation for Selection
A broad-sense heritability estimate is a partial
description of one trait in one group of animals
at some particular time. As such it is not
accurate enough to use as a reliable estimate of
selection potential.
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Heritability Estimation for Selection

• However, VG can be broken down into 3
  subcomponents,
• Additive variance (VA )
• - the average effect of additive components of
  genes
• Dominance variance (VD)
• - deviation from additive components seen in
  heterozygotes that are not precisely
  intermediate to the two homozygous parents
• Interactive variance (VI)
• - deviation from additive components when two or
  more genes behave epistatically (usually minor)

VG VA VD VI
or VG VA VD
or
h2 VA / Vp
h2 VA /VE VA VD
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Effect of Heritability on Selection
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Selective Breeding Can Be Used to Develop
Distinct Populations
Vg
VeVg
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Assigning Phenotype to Continuously Variable
Traits
It can be difficult to assign the phenotype of an
F2 generation individual, because of variation
and closeness of means for P1 and F1. So, how
can we assign phenotype?
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Effect of Testosterone on Adrenal 3bHSD IR-Mass
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Parental and RI Line Phenotypes
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