# Homework Help: Chi square Dominant phenotype problem

1. Feb 26, 2010

### sonyab523

What is the ChiSquare value for the dominant phentotype of the cross Purple versus White. Shouldn't the answer be 696 or 696.75??

Parental Phenotypes

Purple X White

F1 Phenotype

All Purple

F2 Phenotype

705 purple, 224 white

I must be missing something:

Observed Purple=705

Observed White= 224

Total 929

Expected for Purple 705/929=.75

.75x929= 696.75 Purple Expected

Expected White 224/929=.24

.24 x 929= 222.96 White Expected

Second part of the question is

What is the Chi Square Sum (rounding to 3 significant figures) for Purple versus White cross. The answer is 0.391

But when I try to get this answer I use the formula (O-E)^2/E and get .09768 for Purple and .0048 for White which equals .1085

Third part of the question:

Using the following web site: (http://www.graphpad.com/quickcalcs/PValue1.cfm)
What is the P-value (rounding to 3 significant figures) for Purple versus White cross.

Last edited: Feb 26, 2010
2. Feb 26, 2010

### collinsmark

Hello sonyab523,

Welcome to Physics Forum!

Btw, next time you might wish to put this sort of question in the
Physics Forums > Science Education > Homework & Coursework Questions > Other Sciences
category. 'Mostly just introductory physics here.

But anyway, I think I can help (I'm not an expert in genetics, but I'm not completely unfamiliar with it either):

Okay, here is where I believe you went wrong. The Expected White is 0.25 x 929 = 232.25.

In other words, 3/4 of this population is expected to be Purple, 1/4 White.

That makes the (O - E) for White -8.25.

Your Purple calculation is the same as mine. After you make the above correction for White (and recalculate), I think you should be good to move forward.

3. Feb 26, 2010

### sonyab523

I see I forgot to round. I have another question on a similar problem but I'll post start another thread in the right forum. Thank you.

4. Feb 26, 2010

### collinsmark

I was sort of under the impression that it's not a matter of rounding. What I assumed was that Purple and White represent a monohybrid cross between two heterozygotes. This is based more on theory, rather than on the measured statistics of the particular problem. I was assuming that the problem assumes one dominant phenotype (F1 phenotype), and one recessive (F2 phenotype). If my assumption was correct, it seems obvious here that Purple would be the dominant trait, call it "P", and White is the recessive trait, call it "w". Then there are only 4 possible combinations:

PP
Pw
wP
ww

But since Purple is dominant trait, any combination with a P in it will show the characteristics of purple. So PP, Pw, and wP all produce Purple characteristic -- that's an expected 3/4 of the population. Only the ww combination produces a White characteristic. 1/4 of the population.

 Or maybe to put it better, 3/4 and 1/4 of the total number of possible combinations.

Last edited: Feb 26, 2010