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The main blood groups are A and B, but both these groups carry alleles for the O group. It is a recessive trait, as are blue eyes. Just as two parents with brown eyes can have a blue-eyed child, two A+ parents can have an O- child.
The + and - refers to the Rh factor. If a person is Rh+, his offspring can either be positive or negative. However, if a person is Rh-, then the offspring can only be -.
Yes, because both parents could have a recessive gene for the O blood type, and when a person is Rh+ they can either have a + child or a - child. So yes it is very possible.
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