Chapter 19 Flashcards
Discontinuous variation
variation that produces discrete sharply distinguishable phenotypic categories, allowing for consistent phenotype ratios (3:1 or 9:3:3:1) - in single gene traits
Continuous variation
variation distributed across a range of values in an uninterrupted continuum - in polygenic and multifactorial traits
Human height is…
a multifactorial trait with continuous variation
In addition to genetics, human height is affected by ____________ and ____________ factors
developmental and environmental factors - nutrition, exercise, rest
In regard to adult human height, there are more than ____ genetic variants
700
Genetic Potential
is transmitted by parents to offspring, which can be attained if the offspring grows and develops under ideal conditions
Different genes contribute to polygenic traits differently, Major Genes have ______ influence while Modifier (Minor) Genes have _____ influence
greater, less
_______ is a major gene that influences eye color while ______ is a major gene that influences both eye color and skin tone
HERC2, OCA2
Additive Genes
when multiple genes make incremental contributions to a phenotypic value - in polygenic traits where no major gene effects are observed
The alleles of each ________________ can be assigned their own quantitative values of contribution
additive gene
Multiple-gene hypothesis
the hypothesis that 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
To test the multiple-gene hypothesis, Nilsson-Ehle used two additive genes (A + B) that both had two alleles A1, A2, B1, and B2. Alleles ___ and ____ add an equal unit of color to the phenotype while ____ and ____ add no color. The more “__________” alleles a genotype has, the darker the color of what kernels and vice versa.
A1 and B1, A2 and B2, number 1
The number of phenotypes follows the rule “_______” where ______________
2n+1, n = the number of genes
The more additive genes there are that contribute to a phenotype, the ______ phenotypes there are, causing ______ demarcation between categories
more, less
In 1916, East used Nicotiana longiflora corolla length to test the multiple-gene hypothesis. He took pure-breeding long and short corolla lines and measured progeny. His two conclusions were…
(1) the intermediate corolla length in progeny results from segregation of alleles of multiple genes
(2) phenotypic expression is also influenced by environmental factors
T or F: when there is no gene-environment interaction, F2 phenotypic distribution is discontinuous with a predictable 1:2:1 ratio. With moderate gene-environment interaction, there’s a range of phenotypic values with F2 progeny having some overlap. Finally, substantial gene-environment interaction results in F2 progeny that have significant overlap
True
Phenotype = ……
P (phenotype) = G (genotype) + E (environment) + GEI (genotype-environment interactions)
GEI (genotype-environment interactions) is estimated …
only when several genotypes are assayed in several defined environments
Threshold Trait
a polygenic/multifactorial trait that has continuous distribution but can be divided into distinct categories (like affected or unaffected)
Genetic Liability
the idea that certain alleles can push the phenotype toward the affected end of a continuous distribution, each individual’s risk of having the affected phenotype is the result of genotype or in the case of multifactorial phenotypes the genotype and nongenetic influences
Threshold of Genetic Liability
the point when an individual will show the affected phenotype - environmental factors can contribute to reaching threshold
Variance (s^2)
a measure of the spread of distribution around the mean
s^2 = sum [(x,i - x,bar)^2] / df
Phenotypic Variance = ……
VP (phenotypic variance) = VG (genetic variance - genotypic contribution) + VE (environmental variance - environmental contribution)
In parental pure-breeding lines, VP = VE since both parental lines are genetically uniform (VG = 0). In F1, VP = ________ and in F2, VP = _________
VP = VE, VP = VE + VG
Genetic Variance (VG) has three kinds of allelic effects…
*AID
(1) Additive Variance (VA): added effects of all alleles contributing to the trait
(2) Dominance Variance (VD): contributions due to heterozygous individuals not intermediate between the two homozygous states
(3) Interactive Variance (VI): epistatic interactions between alleles of genes
Heritability
the proportion of phenotypic variation that is due to genetic variation
T or F: a high heritability value indicates most of the observed phenotypic variation is due to environmental influences while a low heritability value indicates that most of the observed phenotypic variation is due to genetic variation
False, it’s opposite.
The two measures of heritability are __________________ and ____________.
Broad-sense heritability, Narrow-sense heritability
Regarding the two measures of heritability, what are their respective equations and what do they estimate?
Broad-Sense (H^2 = VG / VP): estimates the proportion of phenotypic variation that is due to total genetic variation
Narrow-Sense (h^2 = VA / VP): estimates the proportion of phenotypic variation that is due to additive genetic variation
Cave Fish case study: if you know the F1 fish are genetically uniform and that VP(F1) = 0.057cm and VP(F2) = 0.563cm, what is H^2?
H^2 = VG / VP
Since F1 is genetically uniform, VP(F1) = VE(F1) = 0.057cm
Since VE(F1) = VE(F2), VE(F2) = 0.057cm
VP(F2) = VG(F2) + VE(F2)
VG(F2) = 0.563cm - 0.057cm = 0.506cm
H^2 (F2) = 0.506cm / 0.563cm = 0.899
Conclusion: roughly 90% of phenotypic variation in eye size between these two populations of blind or sighted cave fish is due to genetic variation
Since identical twins share all their alleles, VP(mz) = _____ but because fraternal twins share half their alleles, VP(dz) = ____________. This means VG = 2 [VP(dz) - VP(mz)]
VP(mz) = VE
VP(dz) = VE + 1/2(VG)
The three caveats of Broad Sense Heritability are (1) Stronger ______, (2) Greater _______, and (3) Greater ___________.
(1) Stronger shared maternal effects in identical twins than in fraternal twins
(2) Greater similarity of treatment by parents of MZ than DZ twins
(3) Greater similarity of interactions between genes and environment in MZ than DZ twins
What does a high narrow-sense heritability value correlate to?
A greater degree of response to selection (artificial or natural selection)
Four attributes of heritability are central to its meaning, what are they?
(1) Heritability is a measure of the degree to which genetic differences contribute to phenotypic variation of a trait
(2) Heritability values are only accurate for the environment and population in which they are measured
(3) Heritability for a given trait in a population can change
(4) High heritability does not mean a trait is not being influenced by environmental factors at all
Quantitative trait loci (QTLs) are genes…
that contribute to phenotypic variation in quantitative traits
QTL Mapping is when you..
map QTLs to chromosome regions/linkage groups
T or F: a chromosome region likely to contain a QTL is identified by frequent co-occurrence of an SNP with a particular phenotype
True
There are two QTL Mapping Strategies: __________ and _________
Genetic Crosses, GWAS (genome-wide association study)
Using genetic crosses to map QTLs, these are the four main steps
(1) construct genetic cross between parental strains w/ different phenotypes
(2) determine DNA markers that differ between parental strains
(3) obtain phenotype and genotype of all progeny
(4) identify associations between phenotype and genotype at individual loci
Using GWAS to map QTLs, these are the five main steps
(1) select a natural population
(2) obtain their whole genome sequences
(3) compares “cases” to “controls” to assess their genotypic difference at each sequence variant
(4) nucleotide frequencies showing significant differences between “cases” and “controls” are candidate QTLs
(5) need to correct for “false discovery rate” (FDR) to reduce false positives
Regarding selection differential (S) and response to selection (R), (h^2) = ________. Ms is ________ and M is _________. Ms - M = selection differential (S). The predicted average for the next generation is ______.
h^2 = R/S
Ms = mean phenotype selected for mating
M = mean phenotype in parental generation
M + R = predicted average for next generation
