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When a person is found to have two variants in a gene, there are two possibilities:
- The two variants are on the same chromosome, or
- The two variants are on different chromosomes.
Two variants on different chromosomes
More commonly, a person has one HBB variant on each chromosome, (called trans, meaning across). Remember, chromosomes come in pairs. In this case, both HBB genes carry a variant. This person might have sickle cell disease, which could range from no symptoms to severe symptoms, depending on which variants are present. When a person has two sickle cell variants (either Hb S or Hb C), the variants will always be in trans.
Two variants on the same chromosome
Less commonly, a person has two HBB variants on the same chromosome (called cis, meaning same side). In this case, the HBB gene on one chromosome has two variants (either Hb S or Hb C along with a variant in another part of the HBB gene). The HBB gene on the other chromosome has no variants. This person is considered a carrier. Learn more about carrier screening