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Polygenic and Multifactoral Traits
• Polygenic inheritance
Continuous variation
Additive alleles
Calculating the number of
• Heritability
Statistical tools: Mean, variance
Broad sense heritability
Narrow sense heritability
Twin Studies and concordance
Continuous variation
• Kolreuter’s cross
• Dwarf x tall tobacco
• F1 intermediate
• F2 intermediate,
normal distribution
Multiple gene hypothesis
• East’s cross of
Nicotiana with
different corolla length
• Indicates Mendelian
segregation of
different phenotypic
• Took subsets of F2
and crossed.
Multiple factor hypothesis
Characters quantified
Two or more genes
Additive alleles
Contribute a constant amount
Non-additive add nothing
All alleles add equally
Polygenic inheritance
• 2 or more genes
• Show continuous
variation vs
• Additive component
• Distinct phenotypic
• Quantitative traits:
size, weight, height,IQ
Polygenic inheritance
Skin color
Additive Model
• (1/4)n= ratio of f2 individuals showing extreme
• n = (2n +1) phenotypic classes
Regression to the mean
• Tendency of offspring
of parents with
extreme differences in
phenotype to exhibit a
phenotype that is
average of the two
parental phenotypes
• Dominant and
additive effects
Threshold effects
• Some individuals show
affected or unaffected
• Predisposition is caused
by a number of genes in
an additive way
• Will develop the genetic
disorder if exposed to
proper environmental
Familial Risks
• First degree: Parentchild share ½ genes
• Second degree:
grandchild share ¼
• Third degree share
1/8 genes
Statistical Analysis
• Mean (X) = ∑Xi/n
• Variance (s2) = (∑Xi – X)2/n-1
• Standard deviation (s) = √s2
Broad-sense Heritabilty
• H2 = proportion of
total variance caused
by genetic variance
• H2 = 1.0, all genetic
• H2 = 0 all variation
due to environment
• Vp= phenotypic var
• Vg= genetic var
• VE= enviromental var
• Vp= Vg + VE
• H2= Vg/Vp
Twin studies
Survey of traits