Quantitative Genetics Flashcards
Measurable in numeric terms.
-Polygenic and multifactorial
Quantitative traits
- Have quantitative traits
- Phenotypes of Q.T. are distributed along a continuum; series of intermediate phenotypes that fall between 2 extremes
ex. Human height, IQ, dispostion, life-span, metabolism
Continuous variation
No degrees of severity
Most traits studied so far that have very distinct phenotypes
Clearly defined by one or a few genes
-In nature, the majority of traits do not follow this mode of expression
Discrete phenotypes
- Controlled by multiple genes and environmental factors
- Each gene is inherited using Mendelian principles
- Modifier genes, differential gene expression, etc. blend together discrete traits
Quantitative Traits
Mode of study that shows inheritance of quantitative traits.
Quantitative Genetics
Number of copies of these alleles influence the degree of severity of a trait
Contributing alleles (a.k.a. additive alleles)
Color of Wheat Kernels Example
- Vary in color from white to dark red
- Alleles A (red) and B (crimson) result in red pigment, while a and b alleles do not produce pigment
One gene variation
F2: 1:2:1 incomplete dominance
increasing Left to right Red and and contributing alleles
Two gene variation
F2: 1:4:6:4:1
aabb: 2Aabb and 2aaBb: AABB
Increasing red and contributing alleles
Three gene variation
F2: 1:6:15:20:15:6:1
increasing red and contributing alleles
More gene pairs result in_______.
“Bell shaped curve”
-Environment “blurs” phenotypic classes.
Samples and Populations
- Too difficult to collect data from every individual for a large population
- Sample a random subset of the population
- Population: entire group of individuals
- Sample: a representative subset
- Sample should be randomly selected and large enough
- Unbiased sample: define the rules before experiment
Summary of continuous phenotype variance for a group or a population
ex. Human birth weight
Distribution
Number to individuals in a sample are plotted (according to phenotype) as a histogram.
Distribution for a quantitative trait
Curve of distribution
Lots of phenotypic variance makes a broader curve
Nature is not:
In opposition to nurture
Nature is:
In correspondence with nurture, influencing one another.
How much variation in a phenotype is due to genetic variation versus environmental variation?
Ex: Superior strains of wheat in a field.
Strains=genetics
Field=environment
In humans, environment influences genetics and subtle choices which influence phenotype.
- Causes variation within each genotypic class
- Causes “blurring” btwn. phenotypic classes in a continuous distribution
- Studied with modern genotyping methods (PCR, etc)
Enviornment affects/quantitative traits
Vp=Vg+Ve+Vg*e
Phenotypic Variance for a trait
genotypic variance contribution of allelic differences at multiple genes
ex. genetic trends in height
Vg
environmental variance contribution from non-genetic sources
ex. impact of nutrition on human height
Ve
genotype-by-enviornment variance
Vg*e
Example: Milk production in dairy cows
- environmental and genetic influences
- How much variation is genetic?
- use this info to see if we should selectively breed the cattle
- We have 4 breeds(<—genotype) of cows
- Collect mild production data from sample of each breed
- Do statistical analysis to determine if any difference is significant.
How much of the phenotypic differences are due to genetic factors.
Heritability
None of the variation in phenotype between individuals results from genetic differences (all environmental)
0 heritability factor
Half of the phenotypic variation is due to genetic differences between individuals and half is due to the environment
0.5 heritability factor
All variation is genetically based
1 heritability factor
Example: Fruit Fly egg size in relation to temp
Observation: over many generations, fruit fly egg size increases in cold temps
Question: How much of the size difference is due to the environment versus genetic differences (evolution)?
Example: Fruit Fly egg size cont…
Vp=Vg+Ve
- Transfer the cold fruit flies to warm (control) temps and mate
- Transfer the control flies to the cold and mate
- Measure egg size of F2 generation (b/c of maternal effect)
- Using stats of egg size measurement, see if there is a difference in the mean egg size of progeny
- The difference between the two is Vg.
What about humans?
Monozygotic twins: phenotypic variance due to Ve
True or False? Traits shared by members of the same family do not necessarily have high heritability.
True.
-They have similar environments.
Individual loci that play a role in quantitative traits.
Quantitative trait loci (QTL)
All QTL and how they interact.
Genetic architecture of a trait.
Study of QTL
Use molecular tools and statistics to link quantitative phenotypes and QTLs.
