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Lecture 6

CAS BI 216 Lecture 6: Genetics - Lecture 6 (2/7/17)
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Department
Biology
Course
CAS BI 216
Professor
John Celenza
Semester
Spring

Description
Lecture 6 – 2/7/2017 – (Chapter 5) o Exam 1 Material • through Chapter 5.3 - except on pages on the three-point cross o Different Kinds of Questions • the ratio is given - hypotheses must fit with the ratio • ratio needs to be derived -> so models can be hypothesized • P-value - -0.025 and 0.025 => 0.05 (5%) of the time · if a p-value is less that 0.05 -> then the hypothesis is wrong o How do we test if data fits a predicted ratio • use a chi squared test -> see if the observed values are significantly different or are just a chance deviation from the prediction o Example • 4:6 -> does it fit a 1:1 ratio · observed 4 and 6 · expected 5 and 5 · get (0-E)^2/E = 0.2 and 0.2 => 0.4 · 0.4 < 3.841 (chi squared) FAIL TO REJECT • Degrees of Freedom · the number of independently derived categories (n-1) · know how many degrees of freedom -> find the cut off • Coin Tosses · all values 2-8 or 8-2 will not reject the hypothesis (chi squared doesn’t work with small values) · 40 head and 60 tails -> but result is different · Total chi squared is > 3.841 · this is why you want to us larger populations if possible • Does data fit a 3:1 ratio (2 categories; 1 degree of freedom) · expected 3 to 1 ratio = 798 and 266 · observed 787 to 277 · run formula -> 0.6 < 3.841 => FAIL TO REJECT · using same data but using 1:1 ratio? -> will reject o For Linked Genes Parental Gametes • non parental games vs. unlinked genes · we can test for unlinked genes - parental games = nonparental gametes · we suppose they are “unlinked” -> if we reject; then they must be linked • Parental: AABB x aabb · F1: AaBb · F1 gametes: AB, Ab, aB, ab · parental - AB and ab · recombinant - Ab and aB • When genes are unlinked: · 1:1:1:1 ratio (F1 gametes) · parental gametes recombinant gametes will be 1:1 • both give different results - because different degrees of freedom · both will reject · you can change your results by changing # of categories and sample size · better to use parentals vs. gametes o The First Genetic Map • looking at parental - at the two ends • what if we did a cross with multiple genes -> do a 3 point cross • see if there is a double recombination in the middle • what if two genes mapped closely to each other? · couldn’t tell exact order - but analyzing all 3 genes helps distinguish
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