week 7 (qualitative traits) Flashcards
define: continuous variation
- characteristics that have large variations
- multifactorial = genetic and non genetic variation affects trait
- dev. and envrt. factors influence the trait
define: polygenic trait
- determined by multiple genes
- different genes contribute differently to phenotype
define: additive gene effects
– multiple genes contribute incremental amount of pheno. influence
explain: multiple-gene hypothesis
- idea that alleles of multiple genes segregate and assort indep. and impart additive effects on pheno.
- ex. kernel colour in wheat plants
⤷ A1 and B1 each contribute colour to a pheno.
⤷ A2 and B2 = no colour
⤷ found that mixing 1s and 2s changed colours (proving additive)
question: formula for number of phenotypic categories? frequency of most extreme phenotype?
number of phenotypes = 2n+1
freq. of most extreme = (1/4)^n
where n = # of genes
question: how to use pascal’s triangle for pheno. freq.?
- n = # of alleles
- triangle numbers = numerators of freq
- total number of combinations = denominator of freq
define: threshold traits
- enough additives push alleles past threshold to be affected
define: genetic liability
- alleles that push pheno. towards threshold
- all the genetic and environmental factors that contribute to the development of a multifactorial disorder
- liability alleles in parents can also be additively passed to offspring
question: formula for phenotypic variance?
VP = VG + VE
phenotypic = genetic + envrt.
**if genetic variance = 0 (ex. twins) phenotype dep. entirely on envrt. -> VP = VE
(vv if envrt. variance = 0, ex. controlled experiments)
explain: variances for parental pure breeding, F1, and F2 progeny
PARENTAL
- VP = VE
- low genetic variation
F1
- VP = VE
- genetically uniform
F2
- VP = VG + VE
- genetically diverse
question: ways to control genetic and envrt. variances?
GENETIC
- inbred populations
- F1 populations
- twins
⤷ siblings = 0.5VG
ENVRT
- controlled experiments
⤷ same treatment, food, water, climate/temp.
question: formulas for broad and narrow sense heritability?
BROAD = H^2
NARROW = h^2
broad = VG/VP
narrow = VA/VP
define: dominance variance and interactive variance
- dominance = contributions due to het. indiv. not having intermediate pheno. between homo. states
⤷ one allele masks the other (dominant) - interactive = epistatic interactions between alleles of diff. genes
question: formula for VG?
VG = VA + VD + VI
(additive + dominance + interactive)
VG also rearranges from VP = VG + VE
define: heritability
proportion of phenotypic variation that is due to genetic variation
- broad and narrow
- both H^2 and h^2 measured from 0 to 1
⤷ 0 = little/no variance, 1 = variance very strongly explained by VG
name: limitations with heritability (4)
- does not indicate mechanism by which genes control a trait
⤷ only measures how much genetic diff. contributes to pheno. - only accurate for the envrt. and population in which they are measured
- heritability can change for a given pop.
- high heritability does not rule out envrt. factors
question: when to use broad sense vs narrow sense heritability
- narrow = only for additive alleles
- broad = for looking at variances
explain: cave fish broad sense example
- crossed blind and sighted cave fish
- measured mean and variance of F1
- measured eye tissue of F2
- assumed that VE for F1 and F2 were the same
- found H^2 was pretty strong
define: selection differential and response of selection
S = difference between mean of whole pop. and breeding pop.
R = depends on extent to which the diff. between pop. mean and mean of mating indiv. can be passed to progeny
⤷ strongest when h^2 = 1
question: formulas for R and S?
S = selectively bred pop. - pop.
R = S(h^2)
define: QTL
- quantitative trait loci
- genes that contribute to pheno. variation in quantitative traits
- used in mapping
- regions are IDd through genetic markers (SNPs)
explain: process of QTL mapping
- construct genetic crosses between parents and diff. pheno.
⤷ to allow recombinances - dev. DNA markers that differ between parental strains
- get a pheno. and geno. of all the progeny
- ID the associations between pheno. and geno.
define: introgression lines
- derived from backcross progeny by selectively breeding inbred lines together
- basically recombinants
⤷ QTL uses markers to locate where the crossovers happen
question: formulas for SD and var?
var = [sum(x - mean)^2] / n - 1
SD = sqrt(var)
