# 20.181/Lecture7

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< 20.181

## Contents |

## quick comment on upPass

- not necessary to find the best tree (you won't be tested on it)
- but here's the correct way to do it:

- (from Peter Beerli's website)

### definitions

- F
_{x}: the upPass set we want to get to

- S
_{x}: the downpass set we got to

- ancestor = a

- parent = p, node we're looking at

- children = q,r

## Revisit overall strategy

- Although up until now we've always started with a tree of known topology, a lot of times you wouldn't know the tree topology beforehand

for all possible trees:return best tree

- compute score (tree)

### Scoring functions

- max parsimony (fewest mutations)
- generalized parsimony (Sankoff: weighted mutation costs)
- Maximum Likelihood

## ML intro

- examples of a ML estimator:
- for normally distributed random var X, X(bar), the mean of the data you observe, is a ML estimator of the mean of the distribution they were drawn from
- A best fit line thru data is a ML estimator.

## Probability Refresher

total area of a box = 1

p(A)= 0.3 , p(B)= 0.3

p(A,B)= 0.1

p(A|B)= 0.1 / (0.1+0.2) = 1/3 = p(A,B) / p(B)

p(B|A) = 0.1 / (0.1+0.2) = p(A,B) / p(A)

With a little manipulation we can derive Bayes' Rule:

p(A|B) = p(B|A) * p(A) / p(B)

## ML in trees

- We are looking for the best tree, given some data. What is the best tree T given the data D?

- p(T|D) is what we want to maximize
- Not obvious how we want to do that... use Bayes Law to rearrange into something we can intuitively understand

p(T|D) = p(D|T) * p(T) / p(D)

- p(D) is a constant ... we don't have to worry about it
- What is p(T), the a priori probability of the tree ?

- Well, without looking at the data, do we have a way of saying any tree is more likely than another one if they don't have any data associated with them ?
- No... not really

- So what we're left maximizing is just p(D|T) and that sounds like a familiar concept!

**NOTE:** Tree now consists of topology AND distances
We ask, what is the probability of *each* mutation occuring along a branch of a certain length? What is the probability that they ALL occurred, to give us the sequences we see today?

p(D|T) = p(x->A|d_1) * p(x->y|d_2) * p(y->G|d_3) * p(y->G|d_4)

p(A U B) = p(A) + p(B) - p(A,B)

p(A <intersect> B) = p(A)*p(B)

- We treat all of these mutations along the different branches as independent events (that's why you multiply the probabilities, because all the events have to happen independently.)

## Jukes-Cantor

- based on a simple cost "matrix"

- probability of changing from one particular nucleotide to another particular nucleotide is 'a'
- probability of any nucleotide staying the same is '1-3a'

if x == y :if x != y :

- [JC eqn you'll derive in the hw]

- [JC eqn you'll derive in the hw]

## Evolutionary Model

gives us likelihood of (D|T) (need branch lengths)

downPass for ML

- compute L(p|q,r,d)

q , r = likelihood of the two subtrees, d are the distances to them