The Chi Square Analysis

1
The Chi Square Analysis
2
Math in Science

• Math formulas can bolster or refute the
  significance of collected data.
• Math makes sense of the numbers

3
Your Offspring (Scary, I know)

• When you perform a dihybrid cross of two
  individuals that are heterozygous for both traits
  you get a phenotypic ratio of 9331
• This is nothing new!
• Example
• Your cross (RrTt x RrTt) to yield 50 offspring
• Round / Teeth ? 28
• Round / No Teeth ? 9
• Square / Teeth ? 9
• Square / No Teeth ? 4

4
Check Your Numbers

• Lets use math to tell us if our data is worth
  anything!
• We are comparing what we actually observed to
  what we expect to observe.
• The alleles were SUPPOSED to separate randomly
  and produce a 9331 ratio
• However, there are other factors at play in
  natural settings.

5
Chi Square Formula
(Observed Expected)2
?
C 2
Expected
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And your number means

• Compare you number to a table of critical Chi
  Square values found in any Statistics book.
• Biologist consider a P value of .05 or less to be
  statistically significant.

7
More about your number
This column is so that were 95 sure we fit the
expected ratio.
Choose n-1 for degrees of freedom where n the
number of possible phenotypes.
8
One more time

• Original hypothesis
• The phenotypic ratio of the offspring from our
  crosses results from completely random allele
  assortment.
• If my Chi Square value is lt 7.82, I can be 95
  sure that the alleles segregated randomly.
  (Accept the hypothesis)
• If my Chi Square value is gt 7.82, I can be less
  than 95 sure the result were random. (Reject the
  hypothesis)

9
You try one

• Use the data from all of the AP sections
  crosses.
• Use the back of your take-home quiz as a guide to
  the calculations.