Genetic background:

CDG-1a is inherited in an autosomal recessive manner. At conception, the theoretical risks to sibs of an affected individual are a 25% risk of being affected, a 50% risk of being an asymptomatic carrier, and a 25% risk of being unaffected and not a carrier; however, based on outcomes of at-risk pregnancies, the risk of having an affected child is closer to 1/3 rather than the expected 1/4.