Recessive inheritance

In a fully penetrant autosomal recessive model with normal allele $A$ and disease allele $a$, $aa$ is affected and $AA,Aa$ are unaffected. A carrier cross $Aa\times Aa$ has offspring probabilities $1/4,1/2,1/4$ for $AA,Aa,aa$, respectively, from the two independent parental meioses.

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

A genetic genetic carrier has one disease-associated recessive allele and one normal allele at a genetic locus, and is unaffected under a fully penetrant recessive model. Two genetic carriers have affected-offspring probability $1/4$ under Mendelian segregation; a genetic carrier and an individual with no disease allele have affected-offspring probability zero under that same mutation-free model.