5.6 Calculation of Chi-square test for deviation from Mendelian ratios

- Put your observed numbers and expected ratios (shall sum up to one) and click the calculation button. In all cells with initial values you can put data (green fields).

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The number of degrees of freedom for the Chi-square is equal to the number of cells with expected values minus one.

Test of segregation ratios from known mating can be done by Chi-square test. If you have a test mating between two heterozygotes Aa x Aa the offspring would have an expected segregation ratio of 1:2:1 as shown in the table below, where the ratios are converted to a 0.25 : 0.5 : 0.25.
Genotype      AA               Aa             aa       Total
Number, obs    30               51             39      = 120 = N
Frequency,     .25              .5            .25      = 1,00
Number,         30               60             30      = 120
Deviations      0                -9              9 
Chi-Square      0               1,35            2,70    = 4,05
To test the applet put the bold face values from the table in the applet and press the Calculate button.

The number of degrees of freedom is df = 3-1 = 2, as the material is only providing the parameter N being used to calculate the expected numbers. The Chi-square value is less than the test value 5,99, which means that there is not statistically significant deviations from a 1:2:1 segregation ratio on the 5% level.

Calculate a Chi-square for the following observation set from test mating of known carriers.
Total numbers of observations   30
Number of affected              16

Is the segregation ratio statistically significant different from a 1:3 segregation ratio ?

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