= Solution
Family comparisons condition on the parental <genotypes>, so cases are compared with the <alleles> they could have inherited. This protects a transmission test from ordinary <population stratification>, avoids having to recruit unrelated controls matched for ancestry, and can resolve <haplotypes> and identify Mendelian inconsistencies. The same <pedigree> may also provide linkage information.
The costs are recruiting and genotyping relatives, missing or unavailable parents, and fewer informative comparisons because <homozygous> parents contribute no transmission contrast. These problems are especially important for late-onset disease. Related cases or multiple siblings also require an analysis accounting for dependence, and <genotyping error> or transmission distortion can mimic overtransmission. Population-based case/control studies are usually easier to assemble and can offer greater power per genotyped person, but need careful control of ancestry and other sources of confounding. Family sampling does not remove every form of <ascertainment in a genetic study> or observation bias.
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