Blood Types Problem Set

Problem 3: What blood type would the father be?

Help to answer the question

If the mother of a child is blood type O+ and the child is A-, what blood type would the father be? Does the Rh factor of the child being - mean that one of the parents has to be negative? Both of my parents are Rh - all of us siblings are Rh- as well. Could two Rh- parents give birth to a Rh+ child? and vice versa? Thanks for all your help. --MaryEllen


Blood types
The father has to be the source of the A allele of a child with blood type A, when the mother is of blood type O. The father could be type A or type AB. The types that can be ruled out for the father are type O and type B.

Rh factor
The Rh factor information doesn't discriminate potential fathers of the child. Even though the child is Rh-, it is not necessary that either parent be Rh-.

The father of the child could be either Rh+ (heterozygous with one Rh+ allele and one Rh- allele) or Rh- (homozygous with two recessive Rh- alleles. )

When the mother is Rh- and the father is Rh+ (heterozygous with one Rh+ allele and one Rh- allele), there is a 50% probability for an Rh+ child and a 50% probability for an Rh- child.

When the mother is Rh- and the father is Rh-, as in the case of your own family, then all children (all of your siblings) will be Rh-.

Once again, if it is important to have correct and highly discriminating information about paternity, my advice would be to explore the possibility of a DNA-based paternity test. Although I cannot personally endorse any particular testing company, you may wish to visit the web page of a paternity lab at

The Biology Project
Department of Biochemistry and Molecular Biophysics
University of Arizona
Thursday, October 23, 1997
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