A question of probability.

roger at kean.ucs.mun.ca roger at kean.ucs.mun.ca
Thu Oct 14 13:14:22 EST 1993


The situation:
	A family is segregating an autosomal dominant trait. There is known 
genetic heterogeneity associated with this trait, but a "good proportion" of 
families show tight linkage to a marker on chromosome 22. The family under 
investigation shows a LOD of 3.0 (or whatever) at theta=0 (i.e. no 
recombinants) using this candidate marker on 22.

The question: 
	BASED ON THE LOD SCORE ALONE (and not on an estimation of the 
proportion of families that are linked to 22 ) what is the probability 
that the trait in this particular family is tightly linked to the marker on 
22?  How does one go about this type of calculation? Any references that 
might help?

	When searching for linkage to a previously unknown locus, a LOD of 
3.0 approximates to a 95% probability of linkage (not the 1000 to 1 
odds frequently cited). How is this changed if we are looking for linkage to 
a single candidate locus as in the present example?

Roger C. Green



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