## usage of chi square in a genetics cross -how does this work?

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MrMistery
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### usage of chi square in a genetics cross -how does this work?

This question is from the international biology olympiad in China, 2005. I have it figured out except for one thing.

Wild type individuals of Drosophila have red eyes and straw-coloured bodies. A recessive allele of a single gene in Drosophila causes glass eye and a recessive allele of a different gene causes ebony body.
A student crosses pure breeding wild type flies with pure breeding flies having glass eye and ebony body and the resulting Fl flies showed all the wild type phenotype for both features. On crossing the Fl flies among themselves the student expect a 9:3:3:1 ratio but

the results are not like that. The actual offspring showed:

Eye Body Number of flies in F2
Wild Wild 164
Wild Ebony 37
Glass Wild 59
Glass Ebony 28
There are two possibilities:
The differences from 9:3:3:1 are coincidental (null hypothesis accepted). The differences do not occur by coincidence (null hypothesis rejected).
You are required to check this applying the x2 (chi square) test.

Ok, i've calculated the value of chi square and got a value of 11,4, which i am pretty sure is correct. But in order to interpret my value i need to know the number of degrees of freedom. How do i calculate that number?

Thanks in advance to anyone willing to help
"As a biologist, I firmly believe that when you're dead, you're dead. Except for what you live behind in history. That's the only afterlife" - J. Craig Venter

canalon
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The number of degree of freedom is usually (Columns+Rows)-1. So i guess in your case that would be 3.
Patrick

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any proof. (Ashley Montague)

MrMistery
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rows in columns in what table? sorry, this is kinda new for me...
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canalon
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Eye x Body in your case.
Your expected table would something like:
Wild Ebony
Wild 9 3
Glass 3 1

To compare with the actual table.
Patrick

Science has proof without any certainty. Creationists have certainty without
any proof. (Ashley Montague)

MrMistery
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Now i get it. thank you
"As a biologist, I firmly believe that when you're dead, you're dead. Except for what you live behind in history. That's the only afterlife" - J. Craig Venter

canalon
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You're welcome. I do too many of those
Patrick

Science has proof without any certainty. Creationists have certainty without
any proof. (Ashley Montague)

canalon
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Obviously not enough (or I simply use too much my computer and I forget how I entered the parameters). Shame on me! The formula to get your degrees of freedoms is:
df = (r - 1)(c - 1)

So in your case the answer was one.

Sorry for the mistake...
Patrick

Science has proof without any certainty. Creationists have certainty without
any proof. (Ashley Montague)

Navin
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Oh - what i was taught to do, was this: (number of phenotypes -1)

Since in your case there is a total of 4 phenotpes in the offspring, then the degree of freedom is 4-1 = 3.

Is this an acceptable method?
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sdekivit
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Navin wrote:Oh - what i was taught to do, was this: (number of phenotypes -1)

Since in your case there is a total of 4 phenotpes in the offspring, then the degree of freedom is 4-1 = 3.

Is this an acceptable method?

no it's not --> the only correct method was mentioned by canalon:

df = (columns-1) * (rows-1)

lara
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yeah it is.take the total no of classes.let it b x. x-1 gives the degrees of freedom.

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