Chromosome Information and Courses from MediaLab, Inc.

Three separate loci (ABO, Hh, and Se) contain the genes that control the location and occurrence of the A and B antigens. Hh and Se genes are closely linked on chromosome 19. The precursor substance is acted upon by the H gene and is converted to H substance. The product of the H gene is an enzyme fucosyltransferase, responsible for attaching fucose to the terminal galactose of the precursor substance on the RBC membrane and thus forming H substance. There are only two recognized alleles at this locus: the active form, H, and an amorph, h. The H gene is a high-incidence gene. People who inherit hh are extremely rare. Since the h gene is amorphic, it does not act on the precursor substance.

Specialists in cytogenetics detect chromosome abnormalities and genetic disorders. Cytogenetics counseling may only be performed by an individual licenses in the cytogenetics specialty at the director level. Specialists in molecular genetics analyze DNA and RNA to find disease-related genotypes, mutations, and phenotypes in order to detect or predict disease and identify carriers. Specialists in histocompatibility test to determine tissue compatibility, prevent infections, and investigate and post-transplant problems. Techniques include blood typing, HLA typing, HLA antibody screening, disease markers, flow cytometry, crossmatching, HLA antibody identification, lymphocyte immunophenotyping, immunosuppressive drug assays, allogenic, isogeneic and autologous bone marrow processing and storage, mixed lymphocyte culture, stem cell culture, cell mediated assays, and assays for the presence of cytokines. Specialists in andrology and embryology examine gametes and embryos, including production, morphology, number, and motility, to address issues of fertility and infertility.

A Barr body appears as a small drumstick-like projection on one of the lobes of a some of the neutrophil in females. Barr bodies are attached to the nuclear lobe by a single narrow stalk which distinguishes them from other thicker projections, sometimes referred to as "clubs." Clubs have a thicker, and sometimes, a double stalk. This projection can be seen in both males and females and has no clinical significance. Barr bodies must also be distinguished from hair-like projections sometimes seen in the band form, following irradiation or in patients with a malignant tumor that has metastasized. Since Barr bodies are the morphological expression of the inactivated X chromosome, one Barr body can be seen in up to 3% of the neutrophils on a female's peripheral blood slide. In rare chromosome disorders in which three or more X chromosomes are present, two to three Barr bodies per neutrophil can be seen. Recognition of a Barr body in a neutrophil is important in order to avoid reporting it as abnormal unless two or more per neutrophil are seen.