HLA haplotypes in dizygotic twin pairs: are dizygotic twins more similar than sibs?

It has been suggested that dizygotic twin pairs share two HLA haplotypes more often than ordinary siblings and thus might be genetically more alike. We tested this hypothesis in dizygotic twin pairs from the Danish Twin Registry. A total of 114 (60 female and 54 male) same-sexed healthy twin pairs aged 18-45 years participated. Dizygosity was established by means of DNA sequencing of nine polymorphic markers. HLA-A, B and Cw specificities were typed with serology, and if data were inconclusive, with DNA typing. If twin partners had the same HLA-types, they were assumed to share two haplotypes. If they had 1 HLA A, B and C antigen group in common they were assumed to share one haplotype and if they had no HLA types in common they were assumed to share zero haplotypes. Since HLA-types from parents were unavailable we could not test for identity-by-descent and thus had a risk of overestimating the number of twins sharing two haplotypes. A Chi-square test was used to compare observed numbers in each haplotype sharing group with the expected numbers. Twenty-nine (expected 28.5) twin pairs had two HLA-types in common, 52 (expected 57) had one HLA-type in common and 33 (expected 28.5) had zero HLA-types in common, p = 0.56. Our data show that DZ twins are not more similar than sibs from different pregnancies in general.