Chapter 19(3) Flashcards
What commonly displays discontinuous variation?
the phenotypes of single-gene traits
What commonly displays continuous variation?
polygenic and multifactorial traits are controlled by many genes and show continuous variation, with a range of values in an uninterrupted continuum
What is an example of a multifactorial trait with continuous variation? Why?
human height
-influenced by multiple genes
-phenotypes are influenced by environmental factors, too, such as nutrition
-parents transmit a “genetic potential” to offspring that may or not be met, depending on the environment
What does continuous variation of polygenic traits result from?
the effects of multiple gene that may exert different amounts of influence
What is a major gene?
gene(s) that strongly influence a trait
What are modifier genes?
genes that influence a trait to a lesser degree
What are additive genes?
the continuous distribution of phenotype in some polygenic traits that results from incremental contributions by multiple genes
What is an additive trait?
a value assigned to each additive gene that indicates its contribution to the polygenic trait
Do all additive genes have largely different values?
No, for certain traits, each of the additive genes has an approximately equal effect, so that no single gene has a major influence on the phenotype
What is the multiple gene hypothesis?
in early 1900s, the idea that segregation of alleles of multiple genes played a role in phenotypic variation
What is the relationship between gene-environment interactions and range of phenotypic values?
The more gene-environment
interaction occurs, the wider the
potential range of phenotypic
values that may occur
What happens when no gene-environment interaction occurs?
genotype corresponds to a distinct phenotype
What are threshold traits?
some traits may have continuous variation of phenotype but the phenotypes can still be divided into distinct categories
What two categories can individuals with threshold traits be?
-“affected” (having the condition)
-“unaffected” (normal)
What is the variance?
s^2
-is a measure of the spread of distribution around the mean; it interprets how much variation exists within the sample
What is genetic liability? What kind of distribution?
the organism’s risk of having the affected phenotype
-continuous distribution
What is the standard deviation?
s
-expresses deviation from the mean in the same units as the scale of measure for the sample
-found by taking square root of variance
What is Vp?
key part of analyzing quantitative trait variation is to analyze the factors thought to contribute to
phenotypic variance
What can phenotypic variance be divided into?
-genetic variance (VG)
-environmental variance (VE)
found by adding VG and VE
What is genetic variance?
the proportion of phenotypic variance due to genotype differences
What three different kinds of allelic effects can genetic variance be divided into?
- Additive Variance (VA): derives from the added effects of all alleles contributing to the trait
- Dominance variance (VD): results from dominance relationships in which heterozygous individuals are not intermediate between the two homozygous states
- Interactive variance( VI): derives from epistatic interactions between alleles contributing to a trait
What does heritability help measure?
the proportion of phenotypic variation that is due to genetic variation
Does heritability differ from trait to trait? Why or why not?
Yes, heritability differs from trait to trait; a trait with high heritability is largely the result of genetic influences, while one with low heritability is mainly attributable to
environmental variation
* Heritability is an important measure of the potential
responsiveness of a trait to selection
What are the two measures of heritability?
- Broad sense heritability (H^2): estimates the proportion of phenotypic variation that is due to genetic variation
- Narrow sense heritability (h^2): estimates the proportion of phenotypic variation that is due to additive genetic heritability
How are both broad sense heritability and narrow sense heritability expressed?
as proportions ranging in magnitude from 0.0 to 1.0
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 and population can change
- high heritability does not preclude environmental factors
How can broad sense heritability be used?
as a general measure of the magnitude of genetic influence over
phenotypic variation in a trait, when VG cannot be partitioned
How can narrow sense heritability be used?
Estimates of phenotypic variation that is due to the effects of alleles of additive genes is particularly
useful in agriculture
What do high narrow sense heritability values correlate with?
a greater degree of response to selection
What is the selection differential (S) and how is it used?
-finding this at start in order to estimate the potential response to selection for a trait
-it gives the difference between the means of the whole population and the breeding population
What does the response to selection (R) depend on?
the extent to which the difference between the population mean of the mating individuals can be passed on to progeny
-R=S*h^2
