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Unformatted text preview: ient of coincidence = 1.2 = 0.75
Interference = 1- 0.75 = 0.25 (interference but low!)
p j r 30.8 m.u.
20.8 m.u. Computed an RF of 20.8 mu between p and j
and an RF of 10.0 mu between j and r
If crossing over is independent between region
1 and region 2, then the probability of a double
cross over between these two regions is
equal to the product of the probabilities of
the two events occurring separately. 0.208 x 0.100 = 0.0208
We would expect 0.0208 double crossovers to occur
However, if look at data set we only had an observed 6/500 flies or 1.2 % where a
double cross over occurred.
So the coefficient of coincidence is = observed DCO frequency/ expected DCO frequency
= 0.012 / 0.0208 = 0.577 - means that only 57.7% of the expected DCO occurred
Interference along this piece of DNA is 1 – 0.577 = 0.423 Testing Goodness-of-Fit to a Genetic Hypothesis:
The Chi Square Test
Hypothesis Testing: General
Geneticists want to know if data they observe from a
genetic cross is in satisfactory agreement with a theoretical
or expected prediction (based on a hypothesis).
Always bound to be statistical variation in observed results from
experiment to experiment from that of expected!
Devised test which decides whether an observed result deviates
too far from an expected result
Called Goodness of Fit -fit means how closely observed results
“fit” with or agree with expected results For example: test cross F1 dihybrid pea (YyRr) with
homozygous recessive (yyrr)
(YyRr x yyrr)
568 progeny with 154 Y_R_:144 Y_rr:124 yyR_:146 yyrr
Hypothesis: 2 gene pairs assorting independently
predicts an F2 ratio of 1:1:1:1 for 4 phenotypes
In theory expected values would be 142:142:142:142
Observed values deviate from values predicted by our
hypothesis, but is this significant (reflecting a real difference
between observation + theory) or insignificant (reflecting random
sampling error or chance)? Question: if proposed genetic mechanism is correct,
what is probability of obtaining results that differ by
this much or more from...
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- Spring '10