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Suppose the two parents of a child have blood groups A+ve and O+ve, and the child has O-ve type. For blood group, there are two alleles. Since the child has O, the father must have one 'A' allele and one 'O' allele.

Does such a reasoning hold for the Rhesus factor too? Should I interpret that both the parents have one + allele and one - allele?

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  • $\begingroup$ Since the Rhesus antigen is inherited in a dominant negative way, both of your parents need to be heterozygote for it, so you can carry -/- for your alleles. $\endgroup$ – Chris Jun 28 '15 at 10:56
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    $\begingroup$ I edited the question to remove reference to self. Good question. $\endgroup$ – AliceD Jun 28 '15 at 13:04
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Rhesus antigen (Rh-D) is inherited in an autosomal dominant pattern. Therefore you are correct in your reasoning.

Simple problems like this are often best worked out using Punnet Squares, shown below for the scenarios you have considered.


Because the inheritance pattern is dominant, if either parent was homozygous (DD) for Rh-D then 100% of their offspring will be Rh+ve.

Punnet square for Rhesus disease with homozygous positive and negative parents

In order to have a rhesus negative child, neither parent must be homozygous for the D allele. You are correct that one possible scenario is a matching pair of heterozygotes:

Rhesus inheritance punnet square for matching heterozygote parents

In this situation 3/4 of the children are rhesus positive and 1/4 rhesus negative.

There is an alternative where one parent is Rhesus negative and the other positive. In this case there is one heterozygous parent and one homozygous negative parent:

Punnet square for rhesus inheritance with one homozygote negative and one heterozygote

In this scenario, half of the children are rhesus positive and half rhesus negative.

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  • $\begingroup$ Thanks. The second case does not arise because the mother is Rh- positive, in my question. $\endgroup$ – Bravo Jun 29 '15 at 12:24
  • $\begingroup$ @Bravo ah of course, I missed that element! $\endgroup$ – Rory M Jun 29 '15 at 14:57

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