With each pregnancy there is a 50% chance that the child will be thal trait (like flipping a coin) because we have two beta genes (1 thal & 1 non thal) - and we pass one of the two to each child. The chances are the same as having a child of the same gender each time (50%).
Not exactly true, at least in some cases not. There are families where for example only boys are born. You know these stories where there was no male child in the royal family - only girls, and such stories.
It's because of the chemical structure of the genes in some families. And the thal trait in your children is because of the chemical structure of the genes - your (and your husband) - seems like your beta thal trait gene wins every time in "chemical battle" between genes during conception (this crossover reaction or something like that - I don't remember now).
So many years from now, when there is complete gene scan available for every human, it will be possible to assess the probability much better, because it will be possible to asses chemical compatibility or incompatibility of father's and mother's genes.
For now our assesing is based on incomplete data, where we assume that every gene combination is equally likely - and it is not.
Regards,
B.