Glucose-6-phosphate dehydrogenase deficiency

G6PD deficiency is inherited in an X-linked recessive manner.^[1] X-linked recessive conditions are much more common in males, who have only one X chromosome (and one Y chromosome). Females have two X chromosomes, so if they have a mutation on one of them, they still have one X chromosome without the mutation. Females with one X chromosome mutation are known as carriers and are usually unaffected. However, females can be affected if they have a mutation in both copies of the G6PD gene, or in some cases, if they have only one mutation. Females with one mutation may have lower G6PD activity than would normally be expected due to a phenomenon called skewed lyonization.^[2]

If a mother is a carrier of an X-linked recessive condition and the father is not, the risk to each child depends on whether the child is male or female.

• Each son has a 50% chance to be unaffected, and a 50% chance to be affected
• Each daughter has a 50% chance to be unaffected, and a 50% chance to be a carrier

If a father has the condition and the mother is not a carrier, all sons will be unaffected, and all daughters will be carriers.

There is nothing either parent can do, before or during a pregnancy, to cause a child to have this condition.

Females can have symptoms of G6PD deficiency. Females can be affected if they have a mutation in both copies of the G6PD gene, or in some cases, if they have only one mutated G6PD gene. 

Depending on how mutations affect G6PD enzyme activity, symptoms in any individual can range from mild to severe. Females with mutations in both of their G6PD gene copies can experience the full range of symptoms similar to affected males (who have one mutation). Females who have one G6PD mutation can also have mild to severe symptoms as a result of a phenomenon called skewed lyonization. This phenomenon is also referred to as skewed X-inactivation.^[2]

Because females have two X chromosomes and males have one X chromosome, one copy of the X chromosome in each of a female's cells is "inactivated." This is to prevent females from having twice as much genetic information coming from the X chromosome. Typically, the "choice" of which X chromosome to inactivate is random, which theoretically would lead to each X chromosome being inactivated around 50% of the time. However, in come cases, the activation is skewed, leading to one X chromosome being inactivated significantly more than the other. If a female has a G6PD mutation on the X chromosome that is significantly more active, she can have mild to severe symptoms of G6PD deficiency. The severity depends on how the mutation affects G6PD activity, and the extent to which X-chromosome inactivation is skewed.