Sickle cell trait
Sickle cell trait (or sicklemia) is the condition brought about by being heterozygous for one abnormal allele of the beta gene HBB gene. [Carrying two rather than one abnormal HBB alleles brings about sickle cell anemia.] Although sickle cell trait (technically referred to as hemoglobin genotype AS) is generally regarded as benign or even advantageous (e.g. increased malarial resistance), it is also associated with significantly higher relative risk - 30 to 40 fold - of sudden death during extreme physical exertion such as that associated with military boot camps and some sports training regimens. However, in absolute terms, the risk is still rather low - 30 per 100,000 vs 1 per 100,000. Wikipedia[PMID 3627196]
Note that genoset gs229 classifies for this condition.