In genetics, which type of trait is expressed only when both alleles are recessive?

A trait that is expressed only when both alleles are recessive is known as an autosomal recessive trait. This occurs when an individual carries two copies of the recessive allele for a given gene. If an individual has at least one dominant allele, the dominant trait will be expressed instead.

For autosomal recessive traits, both parents must contribute a recessive allele for the offspring to exhibit the trait. This means that even if one parent has the dominant trait, as long as the other parent provides a recessive allele, the offspring can be carriers but will not show the trait themselves unless they inherit a recessive allele from both parents.

In contrast, autosomal dominant traits require only one copy of the dominant allele to be expressed, while X-linked traits are associated with genes located on the X chromosome and may have different inheritance patterns depending on the sex of the individual. Mitochondrial traits are inherited differently as they are passed down from the mother to all of her offspring. Thus, the distinctive feature of an autosomal recessive trait is the necessity of having two recessive alleles for expression.