BICD 100
Week 2 Handout: Dihybrid Crosses / Chi Square
Advice
:
•
You should be familiar with the
Mendelian phenotypic ratio
for an
F1 dihybrid cross
(
9 AB : 3 Abb : 3 aaB : 1 aabb)
 however, don’t worry about memorizing the phenotypic ratios for the
nonMendelian
genetic
interaction cases (synergistic, epistasis, incomplete dominance, redundant, etc.)
 if you are presented with a phenotypic ratio that deviates
from 9:3:3:1, think about how it
might have occurred.
This may help in finding a strategy to solving the problem.
Examples: Phenotypic ratio of
15 : 1
This can occur if 9 AB : 3 Abb : 3 aaB are grouped together (9+3+3 = 15)
 How can these 3 classes give the same phenotype?
A and B could be
redundant
genes
Phenotypic ratio of
12 : 3 : 1
This can occur if 9 AB : 3 Abb are grouped together (9+3 = 12)
 How can these 2 classes give the same phenotype?
A could be
dominant epistatic
to B
** refer to Midterm Question #7 and Additional Problem 7. for practice on this**
•
For
Chi Square test
, think about flipping a coin:
 if someone flips a coin 10 times, and gets 5 heads and 5 tails, would you be surprised? (no)
 what about 6 heads and 4 tails? (probably not)
 what about 10 heads and 0 tails? (pretty weird!)
 now, if someone flips a coin 100 times and gets 100 heads and 0 tails, you would think
something’s really messed up about that coin (or the person tossing it)!
 this “weirdness factor” is what Chi Square is testing: if you have a model (such as a coin
being equally likely to land heads vs. tails), what is the probability that your data really fit the
model?
In the last case, the probability would be very low (way below the P = 0.05 cutoff in
a Chi Square test), so you would reject the model.
Perhaps the coin is not identical on both
sides  there could be a magnet on one side, etc.
 try setting up Chi Square tests with different coin flip scenerios to get the concept
* for genetics problems, instead of a coin toss, the model involves alleles
segregating into gametes (which is essentially like flipping a coin, given the
principles of segregation / independent assortment)!
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 Spring '08
 SOOWAL
 Genetics, CHI SQUARE

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