heterozygous beta thalassemia traitbeta thalassemia trait diagnosisbeta thalassemia trait pregnancybeta thalassemia trait meansbeta thalassemia trait carrierbeta thalassemia trait anemiabeta thalassemia minor traitdelta beta thalassemia traitsickle beta thalassemia traitbeta traitthalassemia beta trait adalahbeta thalassaemia trait symptoms