A RH negative father has a RH negative son. What is the possibility of other children being RH negative? May we then conclude that the son's mother is also negative?

1 Answer

It all depends on the genes carried by the mother. If the mother is double recessive, all children become negative; but in this case, son's mother may not be negative.


Having a positive blood type is determined by the presence of the RHD gene, which encodes for the Rh D Antigen on the surface of Red Blood Cells. If that antigen is not present (due to defective genes), then the person has a negative blood type.

The RDH gene is present in 2 copies in our genes. However, just 1 copy is enough to have the Rh D Antigen and have a positive blood type. And vice versa, you would need to have both 2 copies of the genes to be defective in order to be negative blood typed.

So in this example, the father is #Rh(-)#, meaning that he has 2 defective copies of the RHD gene. And the same goes for his son, who is #Rh(-)# as well.

  • From this information, we can conclude that the mother has at least 1 defective gene out of her 2 copies. So if the other copy is defective as well, then she would be #Rh(-)# and hence, all their children will be #Rh(-)#.
  • However, if the mother's other copy of the RHD gene is NOT defective, then it means she is #Rh(+)# and hence there is the chance that she might pass on this working gene to their second child, making that new child #Rh(+)#.

This can be summarized by this illustration:


I hope that answers your question.

Further Reading: