Introduction to Genetic Analysis 134

Introduction to Genetic Analysis 134 - 44200_04_p115-150...

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133 4.5 Using Lod scores to assess linkage in human pedigrees The obtained value of ± 2 (4.97) is used to find a corresponding probability value p , using the 2 table (see Table 2-2). Generally in a statistical test the number of degrees of freedom is the number of nondependent values. Working through the following “thought experi- ment” will show what this means in the present applica- tion. In 2 ² 2 grids of data (of the sort we used above), since the column and row totals are given from the ex- perimental results, specifying any one value within the grid automatically dictates the other three values. Hence there is only one nondependent value and, therefore, only one degree of freedom. A rule of thumb useful for larger grids is that the number of degrees of freedom is equal to the number of classes represented in the rows minus one, times the number of classes represented in the columns minus one. Applying that rule in the pres- ent example gives df ³ (2 ´ 1) ² (2 ´ 1) ³ 1 Therefore, using Table 2-2, we look along the row cor- responding to one degree of freedom until we locate our 2 value of 4.97. Not all values of 2 are shown in the table, but 4.97 is close to the value 5.021. Hence the correspond- ing probability value is very close to 0.025, or 2.5 percent.
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This note was uploaded on 01/10/2011 for the course BIOL BIOL taught by Professor Johnson during the Spring '08 term at Aberystwyth University.

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