Chapter 24 - Quantitative Genetics Flashcards
What are quantiative/continuous traits?
Traits that vary continuously
Exhibit overlapping phenotypes
Tend to be polygenic (many loci for a phenotype) and/or influenced by the environment
What are multifactorial characters?
Characteristics that are both polygenic and influenced by the environment
Differentiate between discontinuous and continuous traits
Discontinuous/qualitative:
- Only have a few phenotypes
- Relationship between genotype and phenotype is straightforward
- Dominance and epistasis may allow for genotypes to produce different phenotypes, but relationship still remains simple
Continuous/quantitative:
- Traits vary continuously
- Overlapping phenotypes
- Relationship between genotype and phenotype is complex
- Same/similar phenotypes may arise from many different genotypes
- Each genotype forms a different phenotype
- As number of loci encoding a character increases, number of phenotypes increases, and differences between individual phenotypes becomes more difficult to distinguish
What happens when environmental factors influence the phenotype?
A single genotype can produce a range of phenotypes that overlap
Overlap can make it difficult to determine whether individuals differ in phenotypes because of genetics or environmental factors
It’s impossible to assign a genotype to an individual on the basis of phenotype alone – tend not be able to use genetic crosses, have to use statistical analyses instead
What are meristic characteristics?
When phenotypes vary in whole numbers, but may be caused by continous genetic variation
What are threshold characteristics?
Only two phenotypes (presence/absence) whose expression depends on an underlying susceptibility that varies continuously
Quantiative because often determined by multiple genetic loci and environmental factors
How does polygenic inheritance work?
The proportion of F2 individuals that resembles one of the original parents can be used to estimate the number of genes affecting a polygenic trait
When 2 homozygous individuals for different alleles are crossed to produce heterozygous F1, the result will be ¼ of the F2 will be homozygous like one of the original parents
The same can occur at 2 different loci, and you would multiply ¼ x ¼ to get 1/16 of F2 being the same as an original parent
(1/4)^n will be the proportion of individuals among the F2 progeny that should resemble the original homozygous parents
* n = number of loci with a segregating pair of alleles
What are the 7 methods to analyze quantitative traits?
Distribution
Samples and populations
Mean
Variance
Standard deviation
Correlation
Regression
What is distribution?
Phenotypic variation in a group can be represented by a **frequency distribution **– number of individuals who fit the different phenotypes
Phenotypes on X axis, number of individuals on Y axis
Many continuous characteristics exhibit bell curve
Can be skewed or bimodal (2 peaks)
What are samples and populations?
Population – group of interest
- Populations tend to be too large to collect data from entire population
Often measure a smaller collection of individuals – called sample
- Use those measurements to describe population as a whole
Good sample must have these characteristics:
- Must be representative of whole population - Done by conducting a random sample
- Sample must be large enough that chance differences between individuals in the sample and whole population don’t distort the estimate of the population measurements
What is mean?
Statistic that provides information on the center of a distribution
AKA: average
What is variance?
Indicates the variability of a group of measurements
How spread out the distribution in
Can have the same mean, but difference variances
What is standard deviation?
Describes variability in group of measurements
What is correlation?
When 2 characteristics are related or not independent of one another, we can determine their correlation
When correlated, a change in one character is likely to be associated with a change in another character
Correlation coefficient ranges from -1 to +1
- +1 = direct correlation - When one increases, the other increases
- -1 = inverse correlation - When one increases, the other decreases
Can be calculated for 2 traits among individuals or 1 trait between pairs of individuals
Limitations:
- Correlation only indicates an association – doesn’t indicate a cause and effect
- Correlation also doesn’t mean that the values of 2 variables are the same, rather it means that the change in one variable is associated with a proportional change in another variable
What is regression?
Predict characteristics of offspring given the characteristics of the parents
Plot points that reflect relationship between traits of the individuals
Fit a regression line that depicts the relationship between the two individuals
- Regression line – line that best fits all of the points on the graph and defines the relationship between the x and y
Can find regression coefficient from regression line – how much y increases on average per an increase in x
As slope of regression line increases, regression coefficient tends to increase
How is stats applied to the tobacco plant experiment?
In parental generation, there are flower lengths that have 2 different means
In F1, flower length is about halfway between that in the two parents and the variance is similar to the parents
- Mean means that genes were additive in their effects and variance means that all F1 individuals have the same genotypes
In F2, mean is similar to F1, but variance is greater
- Variance means that all of the F2 progeny don’t have the same genotypes anymore
What is heritability?
Proportion of the total phenotypic variation that is due to genetic differences
Allows us to differentiate genetic influence versus environmental influence
How is phenotypic variance calculated and what does each variable mean?
V(p) = V(G) + V(E) + V(GE)
V(G) = Genetic variance – components of phenotypic differences that are due to genetic differences among members of a population or sample
V(E) = Environmental variance – components of phenotypic differences that are due to environmental differences
- Can include differences that result from environmental factors and random differences in development that aren’t attributed to any other factors
- Includes any variation that is not inherited
V(GE) = Genetic environmental interaction variance Arises when the effect of a gene depends on the effect of a specific environment where it is found
(x) means subscript
What is the equation to determine genetic variance and what do the different variables mean?
V(G) = V(A) + V(D) + V(I)
V(A) = Additive genetic variance – comprises additive effects of genes on a phenotype that can be summed to determine the overall effect of the phenotype
- Primarily determines resemblance between parents and offspring
V(D) = Dominance genetic variance – alleles in a locus are not additive and rather, the effect of an allele is determined by the identity of the other allele at that locus (recessive masked by dominant)
V(I) = Gene interaction variance – genes at different loci may interact in the way that alleles at the same locus interact
- When genes interact, the effect of those genes are not additive
(x) = subscript
What is the overall phenotypic variance equation (with genotypic variance)?
V(p) = V(A) + V(D) + V(I) + V(E) + V(GE)
What are the 2 types of heritability?
Broad-sense heritability
Narrow-sense heritability
What is broad-sense heritability and how is it calculated?
Must know this equation
Proportion of phenotypic variance that is due to genetic variance
H^2 = V(G)/V(p)
- Value of 0 = all phenotypic variance results from the environment
- Value of 1 = all phenotypic variance results from difference in the genotype
- Value between 0 and 1 = both environmental and genetic factors play a role in phenotypic variance
(x) = subscript
What is narrow-sense heritability and how is it calculated?
Must know this equation
Proportion of phenotypic variance that results from additive genetic variance
h^2 = V(A)/V(p)
(x) = subscript
What are limitations of heritability?
Does not indicate degree to which characteristic is genetically determined
- Only indicates degree to which genes determine variation in a characteristic
An individual does not have heritability
- Only looks at groups of organisms
No universal heritability for a characteristic
Even when heritability is high, environmental factors may still influence the characteristic
Indicates nothing about the nature of population differences in a characteristic
What are quantitative trait loci (QTLs)?
Chromosomal regions containing genes that control polygenic characters
Not a gene, instead is a map location for a chromosomal region that is associated with quantitative trait
How does mapping QTLs allow us to identify QTLs among progeny of a cross?
Mapped by linkage analysis
If the inheritance of a genetic marker is associated consistently with the inheritance of a particular phenotype, that marker must be linked to a QTL
Common way to map is to cross 2 homozygous individuals that differ in alleles at many different loci
- F1 progeny are crossed or back crossed (crossed with parent), which allows genes to recombine
- Unlinked genes will recombine freely, whereas linked genes will recombine together
- Many different combination of alleles appear in F2
- Measure offspring for the quantitative trait and genotype for numerous genetic markers
- A correlation between genotype and quantitative trait indicates that the genetic marker is linked to the QTL
What is response to selection?
The extent to which a characteristic is subjected to selection changes in one generation
Determined by narrow-sense heritability and how much selection there is
When narrow-sense heritability is high, offspring tend to resemble parents
Response to selection may eventually level off and no longer respond to selection over many generations
What is selection differential?
Difference between the mean phenotype of selected parents and mean phenotype of the original population
How much selection is dependent on selective difference of individuals selected as parents – closer the phenotypes of parents that are selected, the stronger the phenotype presence
What is phenotypic correlation?
When two or more characteristics are correlated
Between two or more phenotypes in the same individual
Due to environmental and/or genetic correlations
- Environmental – two or more traits that are influenced by the same environmental factor
- Genetic – genes affecting two traits are associated (typically via pleiotropy – affect of one gene on two or more characteristics)
