Genetic Tests of Hypothesis

We have run an experiment, and would like to know if the gene action fits a specific type of epistasis. Our trait is purple flowers vs white flowers. We suspect the trait to segregate as duplicate recessive epistasis.

Experiment:

Do a dihybrid cross and classify the offspring:

Phenotype     Number of F2 individuals 

Purple                    78 

White                     72 

Total                    150

Analysis of Flower Color

Phenotype 	Observed 	Expected 	(O-E) 	Chi-Square 

Purple             78            84.38         -6.38      0.48 

White              72            65.64          6.38      0.62 

Total             150           150              0        1.10

Genetic Hypothesis

The genetic hypothesis is that the genes follow some specific mode of inheritance, e.g., duplicate recessive epistatis.

We are testing whether or not this genetic hypothesis is supported by the experiment.

Statistical Test

Reject the Hypothesis if your data is far enough away from the hypothesized genetic relationship.

Fail to Reject the Hypothesis if your data in not far enough away from the hypothesized genetic relationship.

Chi Square Analysis

Reject the Hypothesis if the calculated Chi Square is greater than the tabled Chi Square value.

Fail to Reject the Hypothesis if the calculated Chi Square is not greater than the tabled Chi Square value.

Chi Square Table (5% Significance Level)

d.f.              1     2     3     4     5     6     7 

Chi-Square      3.84  5.99  7.82  9.49 11.07 12.59 14.07

Results of the Genetic Experiment

If you reject the hypothesis: There is enough evidence at the 5% significance level to conclude that the genetic hypothesis is incorrect. For this experiment, there is enough evidence at the 5% significance level to conclude that the gene action is not duplicate recessive epistasis.

If you fail to reject the hypothesis: The data are consistent (at the 5% significance level) with the genetic hypothesis (Duplicate Recessive Epistasis).

Another Experiment

We are interested in fruit shape in summer squash. We suspect the mode of inheritance is Dominant X Recessive Epistatis. For this mode of inheritance, we would expect a 9:6:1 ratio among the F2 offspring in a dihybrid cross. We perform a dihybrid cross and obtain the following data: 
Phenotype      Observed 

Disc              827 

Sphere            598 

Long               81 

Total           1,506

Analysis 

Phenotype   Observed    Expected     (O-E)     Chi-Square 

Disc           827       847.13     -20.13        0.48 

Sphere         598       564.75      33.25        1.96 

Long            81        94.13     -13.13        1.83 

Total        1,506     1,506           0          4.27