The polymorphism nt 76 in exon 2 of SC is more frequent in whites than in blacks.

The Scianna blood group system comprises seven antigens encoded by alternative forms of SC. The SC gene also has two polymorphisms in the leader sequence, at nucleotides 54 (C/T, silent) and 76 (C/T, 26His/Tyr) in exon 2, which are not involved in expression of blood group antigens. The nucleotide change at position 76 has an NlaIII restriction enzyme site; thus, DNA samples from 100 Caucasians and 100 African Americans were analyzed for the SC nucleotide 76 change. DNA from Caucasian and African American donors was tested by polymerase chain reaction (PCR)-restriction fragment length polymorphism (RFLP) using the restriction enzyme NlaIII. In selected samples, sequencing of exon 2 was performed. PCR-RFLP results for samples from 100 donors (mostly Caucasian) and 100 African American donors (400 alleles) showed the nucleotide 76T variant had a prevalence of 25 percent in Whites and 5 percent in African Americans. In 11 samples (2 C/C, 3 C/T, and 6 T/T) sequencing of exon 2 confirmed the presence of the expected nucleotides at position 76. The allele frequency in Caucasians was 0.75 for nt76C and 0.25 for nt76T. In African Americans, the frequencies were, respectively, 0.95 and 0.05.

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