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

Genetics as it applies to evolution, molecular biology, and medical aspects.

Moderators: honeev, Leonid, amiradm, BioTeam

Post Reply
User avatar
MrMistery
Inland Taipan
Inland Taipan
Posts: 6832
Joined: Thu Mar 03, 2005 10:18 pm
Location: Romania(small and unimportant country)
Contact:

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

Post by MrMistery » Wed Jul 04, 2007 8:47 am

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

User avatar
canalon
Inland Taipan
Inland Taipan
Posts: 3909
Joined: Thu Feb 03, 2005 2:46 pm
Location: Canada

Post by canalon » Wed Jul 04, 2007 3:14 pm

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

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

User avatar
MrMistery
Inland Taipan
Inland Taipan
Posts: 6832
Joined: Thu Mar 03, 2005 10:18 pm
Location: Romania(small and unimportant country)
Contact:

Post by MrMistery » Wed Jul 04, 2007 5:25 pm

rows in columns in what table? sorry, this is kinda new for me...
"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

User avatar
canalon
Inland Taipan
Inland Taipan
Posts: 3909
Joined: Thu Feb 03, 2005 2:46 pm
Location: Canada

Post by canalon » Wed Jul 04, 2007 9:37 pm

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)

User avatar
MrMistery
Inland Taipan
Inland Taipan
Posts: 6832
Joined: Thu Mar 03, 2005 10:18 pm
Location: Romania(small and unimportant country)
Contact:

Post by MrMistery » Wed Jul 04, 2007 9:40 pm

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

User avatar
canalon
Inland Taipan
Inland Taipan
Posts: 3909
Joined: Thu Feb 03, 2005 2:46 pm
Location: Canada

Post by canalon » Wed Jul 04, 2007 9:43 pm

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

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

User avatar
canalon
Inland Taipan
Inland Taipan
Posts: 3909
Joined: Thu Feb 03, 2005 2:46 pm
Location: Canada

Post by canalon » Fri Jul 06, 2007 3:57 pm

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)

User avatar
Navin
Coral
Coral
Posts: 277
Joined: Thu Jun 09, 2005 2:23 pm
Location: Singapore (Asia Pacific)

Post by Navin » Thu Jul 26, 2007 2:55 pm

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?
Botany is the study of what? Bottoms!

sdekivit
King Cobra
King Cobra
Posts: 586
Joined: Sat Jul 30, 2005 7:16 pm
Location: holland
Contact:

Post by sdekivit » Sat Jul 28, 2007 9:30 pm

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
Coral
Coral
Posts: 131
Joined: Sun Sep 25, 2005 5:10 am

Post by lara » Thu Aug 02, 2007 12:14 pm

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

Post Reply

Who is online

Users browsing this forum: No registered users and 3 guests