Earlier this week the Wall Street Journal carried an article addressing the growing interest in finding health issues related to being a carrier of a recessive genetic disease.
Three diseases were discussed in some detail. Sickle-cell anemia is generally thought of as being very harmful when homozygous but essentially benign when heterozygous. But, it has been known for a while that heterozygotes (called sickle trait) can experience red blood cell sickling (and the accompanying pain and tissue damage) under low oxygen tension. The WSJ journal article points out that such sickling can also occur during strenuous physical exercise; the NCAA even has specific guidelines for extra rest for sickle cell heterozygotes.
An emerging story mentioned in the article is the risks of being a carrier for fragile X, an X-linked disorder which can severely impede mental development. Fragile X is a nucleotide triplet repeat expansion disease, meaning that some males who have a disease allele will have mild or no symptoms but can transmit a more severe form of the disease. Male carriers of these alleles can develop severe neurodegeneration late in life, a condition called FXTAS. Female carriers appear to be at greater risk for anxiety and depression as well as premature ovarian failure.
Other examples mentioned are a greater risk of Parkinson's in Gaucher's disease carriers (at about 5-fold greater risk than the general population) and increased risks of chronic sinus disease and asthma in cystic fibrosis carriers.
Touched on in the article is the fact that many carriers are completely unaware of the fact. Most testing is done if someone is (a) aware of the disease in the family and (b) considering having children. Even then, not everyone is tested. Many of these diseases are rare enough that many carriers could be unaware of the disease being present in the family -- if it never happened to manifest or be correctly diagnosed. Other persons may have missing knowledge about their parentage.
I don't know for certain, but I doubt many of these disease-causing mutations are in the tests used by most personal genetic profiling companies, other than the emerging ones focused on reproductive counseling. The availability in the near future of cheap whole genome sequencing in the near future could lead to huge numbers of people discovering these genetic issues. But, for most recessive diseases we do not know any possible negative effects of carrier status. Much more research will be needed to tease out additional issues.