Unformatted text preview: expected values by just chance
or variation?
or The Chi Square Test
Conventional measure of “goodnessoffit” between
observed and expected values is called chisquare
expected
chisquare Indicates how often an experimentally observed deviation
from a particular hypothesis will occur solely by chance
from
chance
• takes into account importance of sample size
sample
• chi square test is always calculated with actual data
 not % or proportions, requires LARGE sample size
LARGE When geneticists want to determine if two genes
When
are linked, they actually test whether the data are
consistent with a null hypothesis of no linkage
consistent
null
no
• large degree of phenotypic variability if linked
• unlinked will assort independently to produce
50% parental and 50% recombinant in F2
50% If the chi square test shows that the observed
If
observed
data differ significantly from what would be
significantly
expected with independent assortment, the
null hypothesis is rejected genes could be linked Applying The Chi Square Test
Applying
First: state the genetic hypothesis in detail
First:
• (YyRr x yyrr) will yield an F2 ratio of 1:1:1:1 if
genes assort independently not linked
Use the data obtained from a breeding experiment
Use
to determine:
to
the total number of offspring
total
the number of different classes of offspring (phenotypes)
(phenotypes)
and the number of offspring observed in each class
number 568 progeny with 154 Y_R_:144 Y_rr:124 yyR_:146 yyrr
568
154
Total offspring = 568
568
Number of different classes of offspring = 4
Number observed in each class = 154:144:124:146
154:144:124:146 2. Calculate the number of offspring that would
be expected for each class if the null hypothesis
expected
were correct (percentage predicted x total
were
number of offspring)
number
Expected values:
(1/4 x 568): (1/4 x 568): (1/4 x 568): (1/4 x 568)
= 142:142:142:142 The Chi Square Test
The
Deviation from predicted χ2 = Σ
3.
Class (Number observedNumber expected)2
Number expected Y_R_
Y_R_
Number observed = 154
Number expected = 142
=Σ Y_rr
Y_rr
1...
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 Spring '10
 christineDupont
 Genetics

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