## Ratio of Phenotypes

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

BioFan222
Garter
Posts: 1
Joined: Thu Jan 10, 2008 8:16 pm

### Ratio of Phenotypes

Hey,

I have to calculate the ratio of phenotypes.

216 violet flowers/long stems
61 pink flowers/long stems
703 violet flowers/short stems
237 pink flowers/short stems

I have 8.5:30:2.5:7.5

But that is not what my teacher wants, I think.

Can you help me?

MrMistery
Inland Taipan
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lol
703:237:216:61
Are you sure he said phenotypes?
"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

blcr11
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Joined: Fri Mar 30, 2007 4:23 am

### Re: Ratio of Phenotypes

You have phenotypic ratios of 703:237:216:61, just like MrMistery said. That reduces to 11.5:3.9:3.5:1. If this is a standard dihybrid cross where the traits are assumed to be unlinked and assort randomly, you expect to see the ratios of 9:3:3:1. So the question is, are the observed numbers consistent with the expected ones. To find out, you have to do a chi-squared test. You have made 1217 total obsevations. The expected number of offspring for each group is:

(9/16)*1217=684
(3/16)*1217=228
(3/16)*1217=228
(1/16)*1217=76

The chi-squared statistic is calculated for each group as [Expected# - Observed#]^2/Expected# and the four numbers are summed to give the total value—I get 4.475. The observed chi-square is compared to a table of chi-squared-critical values for 3 degrees of freedom. To be significant at the 0.1 level—the sort of “minimal” level to be considered significant—the value needs to be greater than 6.25. The observed distribution of phenotypes, then, is consistent with the expected 9:3:3:1 ratio of phenotypes.

MrMistery
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I hate Chi squares. Thanks for doing the work.
"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

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