Can parents both with positive blood types produce a child with a negative blood type?
It never ceases to amaze me how many people offer opinions on blood type when they have no idea what they’re talking about, even when they know it may directly affect a family. What worries me a bit about this question is the ‘for example’ bit. There are much bigger problems with a child having AB blood if *either* of their parents has O blood than for the +/– thing. Melissa is exactly right about that. Everyone has two alleles (versions of the gene) for their rhesus blood factor. Positive is dominant to negative, so whilst + & + alleles gives you a positive blood type and – & – gives you a negative blood type, + & – shows up as positive. A child takes 1 allele from each parent, so if both parents are + & –, they will have + blood types, but there’s a chance each will give their ‘–’ to the child, giving the child – & – which allows the child to be negative. For two positive parents, the chance of having a negative child is comparable to having a left-hander – not extraordinary. Assumin
