It is hard to say without the context but my thought is that the author is describing a skin sample in humans and is describing a situation where some of the sampled cells are homozygous G or g/g while some of the cells have a g-A mutation so are described as G/A. I do not think that they are describing a G to G situation like insertion or deletion of one of many Gs as this would be described differently

Hello,

Yes this is the best description and I already assumed that, but still not sure! or I am just complicating the whole subject!

Thanks a lot