Maple syrup urine disease (MSUD) is a rare inherited human condition. The allele for MSUD is recessive.

What is a recessive allele?

A recessive allele is a version of a gene that is only expressed in the phenotype if an organism has two identical copies of that allele (i.e., it is homozygous recessive). If a dominant allele is also present, the dominant allele will be expressed instead. For MSUD, an individual must inherit the recessive allele 'n' from both parents (genotype nn) to have the condition.