Rs334

This position causes sickle cell anemia

It is an ambiguous flip and is very prone to confusion

dbSNP uses the minus orientation
 * rs334(A) encodes the normal Hb A form of (adult) hemoglobin.
 * rs334(T) encodes the sickling form of hemoglobin, Hb S.

23andMe tests this snp under the name i3003137 on the plus orientation
 * i3003137(T) encodes the normal Hb A form of (adult) hemoglobin.
 * i3003137(A) encodes the sickling form of hemoglobin, Hb S.

Note that the 1000 genomes project is also reporting this in the plus orientation.

gs228 and gs229 cover these arguably better

Only individuals homozygous for this allele, in other words having the rs334(T;T) genotype, will have sickle cell anemia. The (T) allele appears to have maintained through evolution due to the ~10 fold higher resistance to life-threatening forms of malaria that heterozygotes (rs334(A;T) genotypes) exhibit.

See the SNPedia sickle cell anemia entry for more information.