= Sibling recurrence risk
Sibling recurrence risk is the <conditional probability> that another child has the disease, given an affected sibling. For unaffected parents, complete <penetrance> and a single recessive disease allele, the affected child identifies both parents as <genetic carriers>. Independent <Mendelian segregation> then gives recurrence probability $1/4$, whether or not the parents are related. Incomplete penetrance, phenocopies, different causal loci or affected parents change this calculation.
// Target: biology.bigb
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