It shouldn’t be about blame, of course, in the first place. But, what is incomplete about the diagram below?
When a child is diagnosed with an X-linked recessive disorder, the inheritance pattern is often illustrated with a diagram like this one. Notice where the buck stops.
What could going back one more previous generation reveal? There are (at least) five possibilities.
First, let’s be clear, the mother might not be a carrier to begin with.
There might not have been a genetic alteration previous to the diagnosed child; the mutation (to use the “m-word”) just sprang up suddenly in the early embryotic divisions of the affected fetus, causing the disorder. In that case, the mother is not a carrier. However, this diagram might be used to explain how things usually work, even when the mother has not been identified as a carrier through genetic testing.
A second possibility: the mother does have the mutation, but only in some of her ova, or egg cells. She, also, would not be identified as a carrier because the mutation does not affect the DNA in her entire body. A blood test would not reveal the mosaicism (that means some egg cells are affected, others are not) because the DNA in the rest of her body, and many of her ova, does not code for the disorder.
Further possibilities occur when the mother is identified by genetic testing as a carrier in the usual sense (each pregnancy having 25% chance of an affected son, 25% chance of a carrier daughter, 50% chance of an unaffected child, as illustrated in the diagram). But how does she happen to be a carrier?
That brings us to the third possibility: A “spontaneous mutation” occurred making the mother a carrier without previous genetic history in the family. It is often estimated that in some X-linked recessive disorders, around 30% of affected pregnancies result from such “out of the blue” genetic changes.
Fourth possibility: She was a carrier daughter of a carrier mother. The damaged gene “runs in the family,” although there may never have been an affected son born to anyone before this. Of course, everything said above about the mother, can also be applied in the grandmother’s case.
Fifth possibility: Because a daughter gets an X chromosome from each parent, the mother could have inherited the mutation from her father! Gosh, that’s something you seldom hear!
But yes, in spermatogenesis, mutations occur. If the sperm carrying an X chromosome with an altered gene fertilizes the egg, well, there you have it, a new carrier daughter. No one will suspect, there will be no reason to test, until an affected grandchild of the new carrier’s father is born and begins to manifest symptoms of a disorder that no one knew ran in the family…because it didn’t.
I’m not sure why knowledge of these different scenarios should affect the burden of guilt felt by many mothers who are implicated in genetic inheritance patterns, but I strongly suspect that it does. After all, guilt is an emotion and therefore irrational to begin with. It is easy to say, “It’s nobody’s fault.” But in the case of X-linked recessive disorders, given the way the inheritance has been diagrammed for generations, is it any wonder so many moms feel it all came from them?