If a child has the HLA-B27 marker and one parent has been shown not to have it then yes, the other parent must be positive for it. It should be noted however that most people that are HLA-B27 positive never develop any diseases associated with the gene, so it is possible to have it and never know about it.
What is meant by a gene skipping a generation is that it is actually there in the genome of the "skipped" individual but silent, that is, it´s effect cannot be seen. As mentioned above this is most often the case with HLA-B27. This individual´s child might then inherit the gene and be affected by it. A child cannot inherit a gene straight from a grandparent. A parent must have it in his or her own genome in order to pass it on to the next generation.
My husband has the hlab27 marker and has aynkliosingspondilitis (sorry about the spelling) and his parents do not have the diseases associated with the hlab27 marker. I don't believe they have been tested for the marker though. But his great grandfather had AS.
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