Methylmalonic acidemia or propionic acidemia is an autosomal recessive inherited disorder in which the body cannot break down valine, isoleucine, threonine, and methionine.

Lack of functional copies of the enzymes methylmalonyl CoA mutase, methylmalonyl CoA epimerase and propionyl-CoA carboxylase in an adequate level causes methylmalonic acidemia or propionic acidemia.

What is autosomal recessive inheritance?

In an autosomal recessive inheritance, both parents carry the mutated gene and each cell presents the mutation in both copies of their genes. Signs and symptoms of the conditions are not necessarily manifested in the parent. Every generation of an affected family does not show autosomal recessive disorders.

Autosomal recessive inheritance pattern

Autosomal recessive condition is rare, because chances that both parents are carriers for the same recessive genetic condition are low. Even if both the parents are carriers for the same condition, there is only a 25% chance of passing the non-working gene copy to the baby. These odds remain the same with each pregnancy, despite the number of children they have with or without the condition.

The above figure illustrates an autosomal recessive inheritance. The example below shows what happens when both father and mother are carriers of the abnormal gene. As mentioned earlier, chances of passing the abnormal gene, and causing a genetic condition remains 25%.

Let us take an example of parents who are carriers for the abnormal gene. Either they pass on their normal gene or abnormal gene to their child randomly.

The child is affected by the condition: When both the parents carry the same abnormal gene, there is a 25% (1 in 4) chances of inheriting an abnormal gene and child being affected by the condition.

The child is a healthy carrier: There is a 50% (2 in 4) chance of the child inheriting just one copy of the abnormal gene from a parent. Thus the child remains a healthy carrier, like the parents and is not affected by the condition.

The child is neither a carrier nor affected by the condition: There is a 25% (1 in 4) chance of the child inhering both the normal copies of the genes from the parents. Thus the child will neither be a carrier nor affected by the condition.

All in all, there is a 75% (3 in 4) chance that the child will not be affected by the condition. The probability remains the same in every pregnancy whether the baby is male or female, and possible outcomes occur randomly.