= Incomplete-trio transmission-selection bias
If one parent is <heterozygous> and the other parent's <genotype> is missing, a homozygous child reveals the observed parent's transmitted <allele>, whereas a heterozygous child need not. Selecting only revealed transmissions can therefore bias the <transmission disequilibrium test>. For allele-1 frequency $p$ under random mating and fair transmission, the probabilities of an allele-1 or allele-2 homozygous child from an observed heterozygous parent are $p/2$ and $(1-p)/2$. Among selected homozygous children, the observed allele-1 transmission probability is $p$, not generally $1/2$. A complete-trio analysis avoids this reconstruction rule; methods retaining missing-parent families need an explicit <likelihood function> or a valid conditional adjustment. https://pmc.ncbi.nlm.nih.gov/articles/PMC3297829/[Family-based association tests using genotype data with uncertainty] discusses this bias.
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