Lecture 19 Flashcards
Heritability
The proportion of the total phenotypic variation that is due to genetic difference
-not 100% because environmental factors always play some part
Components of phenotypic variance
Vp=
Vg+Ve+Vge
Vg - genetic variance
Ve - environmental variance
Vge - genetic-environ interaction
Components of genetic variance
Vg =
Va+Vd+Vi
Va - additive genetic variance
Vd - dominance genetic variance
Vi -genic interaction variance
What are the types of heritability?
Broad-sense (H^2 = Vg/Vp; all genetic modifiers)
Narrow-sense (h^2 =Va/Vp; additive effects only)
Heritability by parent-offspring regression
h^2 = b or h^2 = 2b
Both parents [b] or 1 parent [2b]
Heritability by elimination of variance components
(Vp - Ve = Vg)
Correlation coefficient of mono vs dizygotic twins
H^2 = 2(Rmz - Rpz)
If the environmental variance (Ve) increases and all other variance components remain the same, what will the effect be?
a. broad-sense heritability will decrease
b. broad-sense heritability will increase
c. narrow-sense heritability will increase
d. broad-sense heritability will increase, and narrow-sense heritability will increase
A
What are the limitations of heritability?
Does not indicate the degree to which a characteristic is genetically determined
An individual does not have heritability
There is no universal heritability for a characteristic
Even when high, environmental factors may influence a characteristic
Indicates nothing about the nature of population differences in a characteristic
What can mapping quantitative train locki by linkage analysis accomplish?
Help identify genes that help determine differences in quantitative traits
When a marker segregates with a characteristic it means it is closely linked to a locus that influences quantitative trait
Natural selection
Selection that arises through the differential reproduction of individuals with different genotypes
Artificial selection
Selection by promoting the reproduction of organisms with traits perceived as desirable
Factors influencing response to selection =
S
Selection differential
Calculation of response to selection =
R
R=h^2 x S
*remember h^2 = narrow sense heritability
Estimating heritability from response to selection =
H^2 = R/S; realized heritability
Predicting the response to selection =
the extent to which a characteristic subject to selection (interbreeding the extremes) changes in one generation
What are limits to selection response?
Response may level off after many generations
Why does the response to selection often level off after many generations of selection?
After many generations, the response to selection plateaus because of two factors
- genetic variation may be depleted - all individuals in the population now have the alleles that maximize the quantitative trait; with no genetic variation, there can be no selection or response to selection
- even if the genetic variation persists, artificial selection may be limited by an opposing natural selection
How is segregation analysis useful?
useful in the analysis of non-mendelian or complex genetic disorders that may be polygenic or the result of gene-environment interactions
What does segregation analysis estimate?
Estimates the approximate mix of genetic and enviromental factors using information from a series of families identified by the researcher
What does association analysis examine?
Whether affected individuals share the common allele more often than controls
- random selection of affected and control groups
- examination of small areas of genome
In association analysis, determination of markers is more prevalent in what?
Diseased individuals
What is the general difficulty with statistical methods?
Claims often refuted because of lack of consistency in resutls
What are the difficulties in segregation analysis?
Choice of families affects results
Multiple genes analyzed together
What are the difficulties in association analysis?
Small areas of genome examined
Larger areas -> false positives
What is regression?
A mathematical relationship between correlated variables
How is regression used?
Used to predict the value of a variable from the value of a correlated variable
Broad-sense heritability
The portion of phenotypic variance that is due to all types of genetic variance, including additive, dominance, and genic interactions variances
Narrow-sense heritability
Only that portion of the phenotypic variance that is due to additive genetic variance
Heritability is the portion of phenotypic variance due to genetic variance; however, it does not indicate what?
What extent the phenotype itself is determined by genotype
What does heritability apply to?
To populations, not to individuals
Heritability is determined for a particular population in a particular environment at a particular time. Heritability determined for ONE population does not apply to what?
other populations, or even the same population facing different environmental conditions at different period
A trait with high heritability cannot be strongly influenced by environmental factors
True or False?
False
It may still be strongly influenced
High heritability does not mean that differences between populations are due to differences in genotype
True or False?
True
How are genes affecting a polygenic characteristic are located with the use of QTL mapping?
Two homozygous, highly inbred strains that differ at many loci are crossed and then F1 are interbred.
Quantitative traits are measured and correlated with the inheritance of molecular markers throughout the genome. the correlations are used to infer the presence of a linked QTL
