Single Gene traits
- Discontinuous
- Discrete, sharply distinguishable categories
Polygenic and multifactorial traits
Show continuous variation
Genetic potential of Human height
- A multifactorial trait with continuous variation
- Effected by developmental and environmental factors - nutrition, exercise, rest
- Parents transmit a genetic potential to offspring that may or may not be met, depending on various influence
- Adult height >700 genetic variants
Polygenes and differential gene effects
- Null model: each gene contributes the same
- Different genes contribute differently: major and modifier genes (less influence)
- Additive genes: each gene contributes an incremental amount to phenotypic value - alleles of each additive gene can be assigned a value of contribution
Multiple-Gene Hypothesis
Alleles of each of the contributing genes obeyed the principles of segregation and independent assortment and had an additive effect in the production of phenotypic variation
- Two genes, two alleles each
- Alleles A1 and B1 each add one unit to phenotype
- Alleles A2 and B2 add nothing
Number of phenotypes from additive genes
= 2n+1, where n is the number of genes
- more genes, more phenotypes, less demarcation between categories
Frequency of most extreme phenotypes based on number of genes contribution to trait
4 ^ number of genes
Allele segregation in quantitative trait production
Variation in pure-breed strands indicates;
1. Allelic segregation at multiple genes
2. Environmental effects
Effect of the environment on phenotypes
- No gene-environment interaction
- A discrete phenotype for each genotype - Moderate gene-environment interaction
- Minor overlaps between phenotypes in the F2 - Substantial gene-environment interaction
- Wide phenotypic ranges and significant overlaps
GEI
Genotype-environmental interactions
- Estimated only when several genotypes are assayed in several defined environments
Determining phenotype when affected by specific environmental factors
Phenotype = Genotype + Environment + Genotype-environment interactions
Threshold Trait
dichotomous phenotypic trait controlled by multiple genes
- Continuous distribution of genetic liability in general population
- Threshold of genetic liability determines affected vs unaffected
Threshold of genetic liability
The point beyond which an individual will show the affect phenotype
- Environmental factors can contribute to reaching threshold
Goals of Quantitative Genetics
- How much is the phenotypic variation contributed by genetic factors
- How many genes influence the specific phenotypic trait
- How much does each of the genes contribute to the phenotypic variation
- How do genes interact with each other to influence phenotypic variation
- How do genes interact with environmental factors to influence phenotypic variation
Descriptive statistics of quantitative trait analysis
- Mean
- Mode
- Median
- SD
- Variance
Variance
s^2 is a measure of the spread of distribution around the mean
= sum of (xi - x)^2/df
Standard deviation
s = sqrt s^2
Phenotypic variance
Vp= Vg + Ve
- Genetic variance is the genotypic contribution
- Environmental variance is the environmental contribution
What are the 3 allelic effects on genetic variance
- Additive variance: added effects of all alleles contributing to the trait
- Dominance variance: contributions due to heterozygous individuals not having intermediate phenotype between the two homozygous states
- Interactive variance: epistatic interactions between alleles of different genes
Types of quantitative variation in a population
- Phenotypic variation
- Genetic variation
- Environmental variation
- Genotype-environment interaction variations
Heritability
The proportion of phenotypic variation that is due to genetic variation
What are the two measures of heritability
- Broad-sense heritability (H^2): H^2 = VG/VP
- Narrow-sense heritability (h^2): h^2 = Va/Vp
Analyzing phenotypes of monozygotic twins
Share all their alleles
Vp=VE
Analyzing phenotypes of dizygotic twins
About 50% their alleles are shared
VP=Ve +Vg
