Anderson Flashcards
What is the principle of maximum parsimony?
- simplest hypothesis
What kind of hypothesis do you always have for transmission genetics?
- genetic hypothesis
How is hypothesis testing carried out for transmission genetics?
- genetic hypothesis leads to prediction
- in some cases can use observed no.s to give idea of genetic hypothesis
- compare prediction with observed
What is a null hypothesis?
- diff between O and E no.s can be explained by chance alone
- no signif diff between O and E
What is P (p values)?
- probability of obtaining observed deviation, or even bigger, assuming null hypothesis correct
What gives a simple approximation of P?
- chi-squared
What does a p value of greater than 0.05 mean?
- no signif deviation from expectation at 5% level
- if did 20x, expect 1x to get deviation bigger, can accept null hypothesis
- no reason to reject it, can’t say its proven, just failed to prove it wrong
What does a p value of less than 0.01 mean?
- signif deviation from expectation at 1% level
- if did 100x and genetic hypothesis correct, only expect deviation 1x, so quite unlikely result just by chance
What does a p value of less than 0.05 mean?
- signif deviation from expectation at 5% level
Why is there a grey area for p values between 0.01 and 0.05?
- can’t just assume one thing if just below threshold and opp if just above threshold
- chi-squared only aid to thinking about signif, have to weigh up data biologically
Where are qualitative differences in phenotype found?
- in “conventional” Mendelian analysis
- prod by allelic variation at single locus
What are quantitative characters influenced by?
- usually several to many genes and env
- look at real pop (not in lab)
- several oligogenes or many polygenes
What is a polygene?
- 1 member of group of genes contributing to quantitative character, NOT group of genes
What is an oligogene?
- 1 member of group of genes, w/ fewer members but each are making bigger contribution
What is a QTL (quantitative trait locus)?
- section of DNA that correlates w/ variation in phenotype
Why is unlinked always the default hypothesis?
- simplest explanation
What are the 2 types of variation?
- continuous
- discontinuous
What is a meristic character?
- countable quality w/ integer values
When are genes said to act additively?
- when sub of 1 allele for another alters phenotype value by certain amount irrespective of other alleles present at same or other loci
What simplifying assumptions are made in additive model for polygenic inheritance?
- 3 genes
- each making same contribution
- no env contribution
What is the additive model for polygenic inheritance (EXAMPLE)?
- height of hypothetical plant controlled by 3 genes
- a1, b1, c1 each add 1cm
- a2, b2, c2 each add 2cm
- a1a1 = +2cm
- a1a2 = +3cm
- a2a2 = +4cm
- then to all of these:
–> b1b1c1c1 = +4cm
–> b2b2c2c2 = +8cm - no dominance, no epistasis
P1 x P2 = a1a1b1b1c1c1 (+6cm) x a2a2b2b2c2c2 (+12cm)
F1 = a1a2b1b2c1c2 (+9cm)
F2 = 8 phenotypes in ratio 1:6:15:20:15:6:1
What are the general features of the additive model for polygenic inheritance?
- F1 mean exactly intermediate between P1 and P2
- F2 mean same as F1
- variation greater in F2 than F1
- extremes in F2 correspond to P1 and P2
How do extremes in F2 correspond to P1 and P2 (additive model for polygenic inheritance)?
- for n genes proportion F2 as short as short parent = (1/4)^n
- which is a v big no.
- so unless no. genes v small or vast no. in F2 gen, then prob not going to see extremes
Why is scale transformation needed?
- common problem to find non-normal distributions = skewed distributions
- not a big problem just need to define quantitative character in way that will fit, eg. log(score)
- makes curve much more symmetrical so maths will now work for it