What is defined as an autosomal recessive disorder?

An autosomal recessive disorder is characterized by the requirement of two recessive alleles for the disorder to be expressed in an individual. This means that a person must inherit one recessive allele from each parent to exhibit the condition. If an individual has only one recessive allele and one dominant allele, the dominant allele will mask the expression of the recessive allele, resulting in a normal phenotype.

For instance, consider cystic fibrosis, which is an autosomal recessive disorder. An individual must inherit two copies of the cystic fibrosis mutation (one from each parent) in order to have the disease. If they inherit one mutant allele and one normal allele, they will not show symptoms and will be a carrier of the condition but will not be affected by it.

Understanding this definition clarifies the inheritance pattern of autosomal recessive disorders and distinguishes them from disorders that may require only one allele or those influenced by multiple environmental factors.