G6PDD (G6PD deficiency) is passed on the X chromosome.

In most cases only 1 of the parents will have G6PDD, so usually girls (XX) will have 1 affected X chromosome and 1 unaffected X chromosome. So in that case the girl is just a carrier but not affected since the unaffected X-chromosome will still produce the G6PD enzyme.

Since boys (XY) only have 1 X chromosome they are always affected.

So that means your dad is affected and your mom might be affected but is definitely a carrier.

In short, mostly boys are affected.

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Added on July 19, 2011, 2:25 pm

I've been reading loads of info on this but so far haven't found a single reference that it can disappear. It's a genetic deficiency. Your X-chromosome can't simply change.

I guess it's possible they made a mistake while testing you previously or now. Stay alert.