Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2003/iii/paper-40/5/v/solution

Using cases as disease haplotypes and controls as control haplotypes in the model requested here, the chromosome counts estimate
These are conditional marker allele frequencies, not the reverse conditional probabilities . Under independent founder chromosomes, conditional on the father's genotype, the phase has weight proportional to , and has weight proportional to . Their normalized weights are and . This conditions on the model's observed parental genotypes; the omitted disease-allele frequencies cancel in the phase-weight ratio.
Using the two phase-specific pedigree likelihoods already derived,
The normalizing factor is independent of , so maximize . Its derivative gives exactly
Only the smaller root lies in :
On this interval , so this stationary point is the unique maximum. The maximum LOD score, retaining the same linkage disequilibrium frequencies in numerator and denominator, is
This is only a likelihood ratio of about against , hence very weak evidence. The population frequencies are treated as fixed plug-in estimates, as requested; uncertainty in those estimates and possible disease-model differences between controls and normal-allele chromosomes would matter in a fuller analysis.

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