Lab 2: Basic Mendelian Inheritance II Probability in Genetic Analysis

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Lab 2 Basic Mendelian Inheritance
IIProbability in Genetic Analysis

• In genetic inheritance, there is often more than
  one possible outcome (as you saw when you did the
  Punnett squares)
• We can use probabilities to predict how
  frequently specific outcomes will occur

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Lab 2 Basic Mendelian Inheritance
IIProbability in Genetic Analysis

• Coin tossing as an example of using probability
• Two possible outcomes heads or tails
• We expect tails half of the time
• therefore, probability of tails 1/2 or
  P(tails) ½
• What about in 10 tosses of a coin?
• We would expect 5 heads
• Probability (P) x number of trials (n)
• 1/2 x 10 5 tails

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Lab 2 Basic Mendelian Inheritance
IIProbability and Statistics in Genetic Analysis

• But, in real life you may not always get 5
  tails in 10 tosses of a coin
• Such results may be due to normal deviations from
  what you expect to see, or it may be that the
  coin is NOT FAIR
• How do you differentiate between results that
  deviate from your expectations by chance and
  those that deviate because the coin is not fair?
• Do a statistical test!

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Lab 2 Basic Mendelian Inheritance IIThe c2
Test (goodness of fit)

• The c2 goodness of fit test helps determine how
  well your actual outcomes (observationsO) fit
  your predicted outcomes (expectationsE)
• Use coin tossing example in lab manual
• Begin by stating a null hypothesis (Ho)
  meaning the case where nothing unusual is
  happening
• Ex. H0 In 10 tosses of a fair coin, I expect
  5
• heads and 5 tails.

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Lab 2 Basic Mendelian Inheritance IIThe c2 Test

• Carry out the experiment
• Calculate a c2 value for EACH possible outcome
  (in this case the number of heads and the number
  of tails) in an experimental trial
• Sum the individual values to obtain a total c2

(O-E)2
C2 S
E
For coin tossing, c2 (heads) c2 (tails)
Trial 1 Trial 2 Trial 3
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Lab 2 Basic Mendelian Inheritance IIThe c2 Test

• Then, determine the number of degrees of
  freedom
• On the c2 table, locate the degrees of freedom
  column
• For this row, locate where the c2 value obtained
  for each trial falls.
• Find the corresponding P (Probability) value. If
  your c2 value falls between two P values, note
  both.

df number of possible outcomes - 1 2
possible outcomes (heads, tails) -1 1
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Lab 2 Basic Mendelian Inheritance IIThe c2 Test

• The P value represents the value that will
• cause us to reject the null hypothesis

• If the P value is

Greater than 0.05, Ho is not rejected (accepted)
Less than or equal to 0.05, Ho is rejected
Report final result in terms of a P value (for
example, p lt 0.01) and acceptance or rejection of
H0
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Lab 2 Basic Mendelian Inheritance IIThe c2
Table
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Lab 2 Basic Mendelian Inheritance IIMaize and
Mendelian Inheritance

• Maize is a convenient model organism for studying
  Mendelian genetics because each kernel is an
  individual progeny
• As with Drosophila, different maize lines can
  be crossed to produce standard Mendelian
  phenotype ratios in the F2 generation

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Lab 2 Basic Mendelian Inheritance IIMaize and
Mendelian Inheritance

• F2 cobs representing monohybrid (single locus)
  vs. dihybrid (two loci) crosses
• Independent assortment (two loci)
• Basic Mendelian F2 ratios from a monohybrid cross
  (1 allele dominant, one recessive)
• 31
• 11
• Basic Mendelian F2 ratios from a dihybrid cross
  (two loci, 1 allele dominant and 1 recessive at
  each locus)
• 9331
• 1111

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Crosses producing a 31 phenotypic ratio in the
F2 generation
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Crosses producing a 9331 phenotypic ratio in
the F2 generation
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Lab 2 Basic Mendelian Inheritance IILab
Exercises

• Use the c2 test on all possible coin toss
  results
• Formulate null hypothesis
• Accept or reject Ho based upon test results

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Lab 2 Basic Mendelian Inheritance IILab
Exercises
Use the c2 test to assess if F2 corn cobs adhere
to predictions based on Mendelian
inheritance Formulate Ho for a monohybrid and a
dihybrid F2 cob based upon simple Mendelian
ratios Count and phenotypically classify all
kernels in each cob Perform c2 tests on each set
of results to see if they follow expectations
derived from simple Mendelian ratios Accept or
reject Ho based upon test results
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Lab 2 Basic Mendelian Inheritance IILab Summary

• Purpose Statement
• Materials and Methods
• Results
• Coin toss and c2 test results
• Monohybrid and dihybrid F2 cob kernel counts and
  c2 test results
• Conclusions
• Analysis of c2 test results Were Hos accepted
  or rejected? Why?
• Answers to questions in lab manual