CHAPTER 19 Flashcards
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
Narrow sense heritability
The proportion of phenotypic variation due to additive genetic variation (h^2 = Va/Vp)
- High NSH correlates with a greater degree of response to selection
What are four attributes of heritability that are central to its meaning
- It is a measure of the degree to which genetic differences contribute to phenotypic variation of a trait
- Heritability values are accurate only for the environment and population in which they are measured
- Heritability for a given trait in a population can change
- High heritability does not preclude environmental factors
Common sources of error in twin studies
- Stronger shared maternal effects in identical twins than in fraternal twins
- Greater similarity of treatment of identical twins than of fraternal twins
- Greater similarity of interactions between genes and environmental factors twins than in fraternal twins
Selection differential
The difference between the population mean value and the mean trait value for the mating population
Response to selection
The extent to which difference between the mating trait value and the population mean value can be passed on to progeny
Heritability and responses to natural selection
Higher heritability, closer in value R and S making mean of offspring resemble mean of mating population
Quantitative Trait loci
Genes that contribute to phenotypic variation in quantitative traits
GTL mapping
Mapping QTLs to chromosome regions/ linkage groups
GTL mapping strategies
- Analyzing genetic crosses
- Genome-wide association studies
Analyzing Genetic Crosses
- Construct genetics crosses between parental strains with different phenotypes
- Develop DNA markers that differ between parental strains
- Obtain phenotype and genotype of all progeny
- Identify associations between phenotype and genotype at individual loci
Genome-wide association studies for QTL Mapping Strategy
GWAS try to identify whether a particular sequence variant is associated with a specific phenotype in a natural population
1. Select a natural population and obtain their phenotype data
2. Obtain their whole-genome sequences
3. Compare “case” and “controls” to assess their genotypic difference at each sequence variant
4. Nucleotide frequencies showing significant differences between “cases” and “controls” are candidate QTL
5. Need to correct for “false discovery rate” to reduce false positives
