If you are both A positive for instance it is possible for a child to have O positive or A positive. If you both are B blood type you will either have B or O,....and so on and so on........
My sister is rhesus negative, but I am O, my daughter is rhesus negative my husband is O too. I'm not sure that it was my Dad who carried the rhesus type blood.
That's why you have to be very specific when you make some types of bets. The OP mentioned "blood group", and the most common type of group is of course the ABO group. When you start to bring positive and negative into it you are talking about Rh(esus) positive and Rh(esus) negative: these are blood types, not groups. Therefore, if the bet was about blood groups only A, B, O, and AB are involved in it: not Rh positive or negative.
I have just look it up on you know where, and it seems that that distinction between "group" and "type" which I mentioned is no longer in vogue. It certainly was when I was a hematology and immunohematology lab tech. Mind you, that was many moons ago. Hey, you learn something every day.
Sorry been wandering among the threads and getting my supper ready. I'm O positive, not sure about MrAsk is pos or negative.
Someone mentioned eye colour. My mother and I have brown eyes and two of my siblings have, but my next eldest sister has blue eyes Rh.pos bloodtype the same as my Father.
MrAsk has blue eyes and my daughter (Rh pos) has blue eyes. I always thought that brown eyes were dominant.