For a child to be O- blood type, do BOTH the parents HAVE to be O- as well?

Question

I am asking because I know a family of four siblings who are all O- blood type and share the same mom, but they don't know if all four share the same Dad (they know two do, but they aren't sure whether the other two are from a different father or the same.... complicated adoption thing). They plan on eventually doing DNA testing, but I thought O- is a pretty rare blood type, and that both parents had to have it for the kids to as well.

So anyone remember their blood type biology units??

Answer 1

No.

O- is recessive, so if both parents are heterozygous with O-, they may produce a child with the O- phenotype (genotype O-O-)

For example:

A+O-  +  B+O-  =  O-O-

Answer 2

both parents dont have to be O neg. only one parent does and the child can pick up the neg from that parent. O neg isnt that rare but its not as common as the rest.  

Answer 3

Nope.  O is recessive which means you do have to inherit one copy of the gene from the mother and one copy from the father.  However, the mother or father can be a carrier for the O blood type and have one copy of the gene but also have a copy of the gene for the A or B blood type.  Since O is recessive, the parent would then be either A blood type or B blood type.  

The only time you know for sure that somebody isn't the parent of a child with an O blood type is if they have the AB blood type.  In that case, they have a gene for the A blood type and a gene for the B blood type.  That doesn't leave them any more room to be a carrier for the O blood type.

I hope that makes sense.