Each parent of a child with MSUD carries a defective gene for MSUD along with a normal gene. The defective gene is a recessive gene, therefore parents are called "carriers" and are not affected by the disorder. Each child with MSUD has received a defective gene from each parent.

When both parents are carriers, there is a 1 in 4 chance with each pregnancy that the baby will receive a defective gene from each parent and have MSUD; a 2 in 4 chance the baby will receive one defective and one normal gene, thus becoming a carrier of MSUD; and a 1 in 4 chance the baby will receive two normal genes. Persons with two normal genes cannot pass MSUD to their offspring.