Question: How many people are affected by methylmalonic acidemia with homocystinuria ?
Answer: The most common form of the condition, called methylmalonic acidemia with homocystinuria, cblC type, is estimated to affect 1 in 200,000 newborns worldwide. Studies indicate that this form of the condition may be even more common in particular populations. These studies estimate the condition occurs in 1 in 100,000 people in New York and 1 in 60,000 people in California. Other types of methylmalonic acidemia with homocystinuria are much less common. Fewer than 20 cases of each of the other types have been reported in the medical literature.
Source: GHR

Question: Is methylmalonic acidemia with homocystinuria inherited ?
Answer: Methylmalonic acidemia with homocystinuria is usually inherited in an autosomal recessive pattern, which means both copies of the gene in each cell have mutations. The parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but they typically do not show signs and symptoms of the condition. When caused by mutations in the HCFC1 gene, the condition is inherited in an X-linked recessive pattern. The HCFC1 gene is located on the X chromosome, which is one of the two sex chromosomes. In males (who have only one X chromosome), one altered copy of the gene in each cell is sufficient to cause the condition. In females (who have two X chromosomes), a mutation would have to occur in both copies of the gene to cause the disorder. Because it is unlikely that females will have two altered copies of this gene, males are affected by X-linked recessive disorders much more frequently than females. A characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons.
Source: GHR