Back when I was going to school, I learned that on average every human being is a carrier for about 8 different genetic defects. That was a long time ago, long before many modern advances in genetics including mapping of the human genome. We now know that most people are carriers of at least 400 genetic variants, and this number is likely to increase as genetic science progresses.
Another study found that about 25% of all individuals are carriers for at least one genetic disorder and almost 6% were carriers of two or more. These percentages also are expected to increase as science progresses. Most of us, however, will not find out about these deleterious gene variations unless our children or grandchildren happen to be affected. So, while we tend to think about people being carriers or not, the real categories may be between who KNOW or don’t know if they are carriers.
Lazarin GA, Haque IS, Nazareth S, Iori K, Patterson AS, Jacobson JL, Marshall JR, Seltzer WK, Patrizio P, Evans EA, Srinivasan BS. An empirical estimate of carrier frequencies for 400+ causal Mendelian variants: results from an ethnically diverse clinical sample of 23,453 individuals. Genet Med. 2012 Sep 13. doi: 10.1038/gim.2012.114.