Genetics are an interesting thing and I am certainly no expert, however interesting things are continually being discovered especially since the human genome project has been completed..
So often we hear carte blanche statements made with such dogmatism by medical people such as, "a CAH carrier will not have symptoms" - end of story. However we know that in a number of (obligate) heterozygous carriers they do have mild and sometimes troublesome symptoms.
The answer possibly lies in what are called 'modifier genes'. These are other parts of our individually unique DNA makeup that may, to put it very simply, 'play down' or 'exaggerate' other parts of our gene expression.
An editorial accompanying an article in the New England Journal of Medicine describes how modifier genes are different from susceptibility genes:
"Susceptibility genes and modifier genes are two biologic phenomena that few clinicians should ignore in the genome era. Susceptibility genes, which are genes with functional variants that affect the causes of disease, are routinely being identified for simple mendelian diseases and, more recently, for common genetic disorders. Modifier genes are distinct from susceptibility genes, in that they are genetic variants that affect the clinical manifestation of disease (as opposed to liability)…The identification of gene modifiers and their interactions in cystic fibrosis and other diseases has only just begun."
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This could explain for example why my mother, who had two CAH children, showed absolutely no signs herself of being a carrier. Whereas her sister, my aunt, who had no CAH children, showed many signs of possibly being a CAH carrier. Perhaps my aunt had other modifier genes that worked for or against a susceptible CAH gene to produce mild symptoms. It is unprovable in the case of my aunt, as she is now dead, but I often wonder.
