Bardet-Biedl syndrome (BBS) has an autosomal recessive pattern of inheritance. People with this syndrome have pathogenic variants (mutations) in both copies of a gene associated with Bardet-Biedl syndrome. These people typically have unaffected parents who each carry a single copy of the mutated gene (and are referred to as carriers). When two carriers of an autosomal recessive condition have children, each child has a 25% (1 in 4) risk to have the condition, a 50% (1 in 2) risk to be a carrier, and a 25% chance to not have the condition and not be a carrier (i.e. have 2 normal copies of the gene). An affected individual's full siblings who do not have Bardet-Biedl syndrome have approximately a 66 % (2/3) chance of being a carrier for the condition. Autosomal recessive disorders are typically not seen in every generation of an affected family. 
Some cases of Bardet-Biedl syndrome appear to require the presence of at least three mutations for the symptoms to show (phenotype), which is known as triallelic inheritance, in which mutations in more than one gene are involved in a condition. In these cases, in addition to inheriting a single mutation of the same gene from each parent, like in a recessive trait, a child also needs at least one more mutation of another gene to have the syndrome.
However, in practical terms, these families are fewer than 10% of all families with BBS. Therefore, the inheritance is considered to be autosomal recessive in most cases.
Last updated: 7/18/2016
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