A1258690176voPWb - PowerPoint PPT Presentation

Title: A1258690176voPWb


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• Quantitative Genetics or
• Polygenic Inheritance
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• One phenotype is determined by multiple genes.

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Some terms     Qualitative traits phenotypes
differ by quality. Exp. red/white eyes, wrinkled
/round peas.   Quantitative traits phenotypes
differ in quantity height, weight, shades of
skin color.   Polygenic inheritance more than
one genes operating to make a single final
phenotype.  
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Phenotypes in qualitative inheritance are all
distinct from each other - discontinuous
variation. Traits in quantitative inheritance
are not as easily to be categorized into distinct
classes - continuous variation.    
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Josef Kölreuter in early 19 century crossed tall
and dwarf tobacco plants F1 all intermediate
in height F2 showed continuous variation in
height from tall to short
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F2 phenotypes showed normal distribution or
bell curve distribution.
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William Bateson (late 1800s - early 1900s)
Multiple-factor or multiple-gene hypothesis
(the earliest form of polygenic inheritance
) Quantitative traits were controlled by
Mendelian genes. Multiple genes contribute to
the same phenotype in a quantitative way.  
(Quantitative traits are controlled by multiple
genes acting together.)
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Example three genes are involved in plant
height. AABBCC 20 cm height AaBaCc 15 cm
height aabbcc 10 cm height.  
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How to relate quantitative traits with Mendels
theory?  Nillson-Ehle (1909) ran a cross between
red wheat and white wheat F1 All plants showed
intermediate color. F2 continuous variation in
kernel colors between red and white.
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Among the continuous traits, about 1/64 showed
complete white one of the parent traits.
1/64 showed dark red another parent trait.
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1/64 ?!
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1/64 matches with the phenotype ratio produced
by the combination of three recessive gene in
a trihybrid cross.
Conclusion Quantitative inheritance also
follows Mendels rule for inheritance The
wheat color is a quantitative trait governed by
three pairs of genes.
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The cross of white and red wheat involves three
gene pairs r1r1r2r2r3r3 x R1R1R2R2R3R3
The darkness of red color is proportional to the
number of R. The darkest class, with 6 R
alleles shows the same color as the dark red
parental.
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This means the effect of allele R is additive
additive alleles, another feature of
quantitative traits.
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A mathematical rule can be developed between the
number of the phenotypes and the genes
involved Number of phenotypes (genes 1) x
2   You can predict the number of the phenotype
by looking at the number of the genes operating
(if its known) 2 genes yield 5 phenotypes 3
genes yield 7 phenotypes 4 genes yield 9
phenotypes  
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For exp., if two gene pairs were involved, five
F2 phenotypes in a 14641 ratio would be
expected.
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Frequency for each phenotype Majority of the
individuals will express intermediate phenotype,
very few will express the extreme phenotype
normal distribution or bell shape.
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Effect of the environment   Most quantitative
traits are subject to environmental effects,
which can cause individuals with same genotypes
to have slightly different phenotypes.
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  Number of genes involved in a quantitative
inheritance can also be estimated
(1/4)n ratio of F2 individuals
expressing either extreme phenotype (if
its known)  
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In Figure 5.3, 1/64 of the F2 wheat kernels
showed two extreme colors of the phenotypes,
white or dark red.   (1/4)n 1/64, n 3
  So, the kernel color of the plants is
determined by three genes.
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The contribution of multiple genes in a
quantitative trait is additive. How to estimate
the contribution of each gene? For example,
plants with genotypes as A1A1B1B1C1C1 produce 7
cm height, Those with A2A2B2B2C2C2 produce 5 cm
height. (Sometimes, one of the extreme
genotypes produces a base value or base line for
the phenotype. They don't produce 0 cm height.)
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How much does each "1" allele contribute? 1).
Difference between the two extremes 7 5 2
cm 2). This difference is caused by 6 additive
alleles 3). Therefore, each additive allele
causes increase of 2/6 0.33 cm above
the base height.
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To predict the height of a plant with
genotype  A1A1B2B2C1C2 5 3(0.33) 5.99 cm
height.   But, there are environmental effects