Introduction: Dombrock blood group system consists of 2 antithetical antigens – Doa (DO 001) a Dob (DO 002), and 3 high frequency antigens: Gya (DO 003), Hy (DO 004) a Joa (DO 005). DO glycoprotein is attached to the RBC membrane by a GPI-anchor. It is a member of mono-ADP-ribosyltransferase family. DO gene is located on the short arm of chromosome 12. It consists of three exons distributed over 14 kB of DNA. Ten alleles of DO gene have been identified. High frequency antigen Gya was discovered in 1967 by J. Swanson et al. Gy(a-) phenotype was found in 1967 and 1968 in two families of Czechoslovak origin. Four molecular mechanisms have been found for this phenotype. The Gy(a-) phenotype is the null phenotype in the Dombrock system. Case report: We found a woman (V.F.) with an antibody against a high frequency antigen, which was identified as anti-Gya and V.F. cells as Gy(a-). We tried to explore possible relationship of V.F. to original probands from 1967 and 1968. We were provided with data of the two families for our search (personal communication). We also investigated V.F.’s brother, daughter and cousin. Methods: Serological investigations were performed using microcolumn system DiaMed ID. DNA analysis: DNA from blood specimens was purified and PCR was performed. PCR products were sequenced. Genealogical searches were performed using ancient church records and official records on emigration to the USA in 19th century. Detailed searches were performed on V.F.’s family and on the first American family (SK). As for the second family, we performed genealogical searches only on names existing in that time in V.F.’s home region. Results: Serology: V.F.: Do(a-b-); Hy- ; Gy(a-). Anti-Gya. Antibody reacted 2+ in IAT, ± to 2+ in bromelain test. Brother: Do(a-b+); Hy+; Gy(a+), but weaker than usual (probably heterozygous status Gya). Daughter: The same blood group ABO as in V.F., reaction of her RBC with V.F.’s serum positive 2+ in IAT, negative in bromelain test. Genomic DNA analysis: V.F.: Revealed a homojednotlizygous single nucleotide mutation IVS1-2a>g in the acceptor splice site of intron 1. This mutation is known to result outsplicing of exon 2. Sequencing specified DOB allele (378 T; 624 C; 793 G). Other three known mutations were ruled out. Cousin: No mutation in DO gene was found. Genealogical searches: No direct common ancestors of V.F. and American families were found, but ancestors of both V.F. and SK’s family lived in the beginning of the 19th century in the same village. Two names or names of their spouses from the second family were found in the same village, but we cannot confirm their identity. Conclusions: The mutation in DO gene in V.F. is the same as in three Gy(a-) persons discovered in 1967 and 1968. In V.F.’s and SK’s family’s ancestors’ home village consanguineous marriages were probable, because the village has been existing since 14th century and about 2 or 3 hundred people used to live there together for centuries. According to our findings we consider ancient consanguinity between V.F. and at least SK’s family very likely.

        Key words: Dombrock, Gregory, phenotype Gy(a-), serology, DNA analysis, PCR, sequencing, genealogical searches