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Chapter 13

HMB265H1 Chapter Notes - Chapter 13: Human Height, Heritability, Phenotype

Human Biology
Course Code
Maria Papaconstantinou

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HMB265 Lecture 13 READINGS - Quantitative Trait Analysis
12.3 Heritability: Genetic Versus Environmental Influences on
Complex Traits
Complex Traits: Influenced by many factors
- Multiple genes
- Interactions between alleles of different genes
- Environmental variation
- Genes x Environment
- EX: human height
- = genotypic information doesn’t always show what phenotype is
Variance is a statistical measure of the amount of variation in a population
Quantitative traits: Phenotype measured over rand of numbers
- = phenotypic values/trait values
- Often have bell-shaped “normal” distribution of phenotypic values
- More genes in expression of trait = more possibilities for phenotypic variation
- = phenotypes of individuals of one genotype distributed in narrow bell-shape curve
- Centered around average phenotype for that genotype
- This b/c individuals w/ same genotype subject to slightly different microenvironments
How to estimate heritability of complex traits?
- I.e. contribution of genes (not environment) to phenotypic differences
- Need numerical description for curve
- Compare individual in population to average phenotype for population
- = total phenotypic variance
- = average squared difference between each individual value and mean
Genetic variance can be separated from environmental variance
Distinguish environmental from genetic effects on phenotypic variation
- Quantify one variable and control for the other
Find effect of environment: Plant genetically identical dandelions
- Variation in stem length = environment
- = environmental variance (VE)
Find effect of genotype: Plant seeds from different plants in controlled greenhouse
- Variation in stem length = genes
- = genetic variance (VG)
Find total impact on phenotype of variation in genes AND environment
- Take seeds of different plants, grow them on common hillside
- Total phenotypic variance:
VP = VE + VG
- VP bell-curve much wider b/c includes both environmental and genetic variation
Heritability is the proportion of phenotypic variance due to genetic variance
Heritability: Proportion of phenotypic variance (Vp) from genetic variation (VG)
- This is broad-sense heritability (H2)
H2 = VG / VG + VE
= VG / VP
- If know ⅔ variables can find unknown
- H2 ranges from 0 to 1
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