Frequency of leukochimerism in Holstein and Jersey twinsets. 
The current study was conducted to test breed difference in the frequency of leukochimerism.  This study used leukochimerism as evidence of placental vascular anastomosis formation and compared its frequency in the Holstein and Jersey breeds.  We test the null hypothesis that there is no difference in incidence of leukochimerism in the Holstein and Jersey breeds.  Hair and blood samples were collected from 85 Jersey twinsets and 80 Holstein twinsets, ranging in age from 1 d to 8 yr.  An additional 7 Holstein twinsets (6 complete and 1 partial where 1 twin died) were sampled originally 48 to 72 h after birth and resampled at 5 to 10 mo of age to provide an assessment of whether leukochimerism changed with age.  DNA was extracted from white blood cells (potentially chimeric) and hair follicles (not chimeric).  DNA samples were successfully genotyped for 19 SNP selected for high minor allele frequency in both breeds based on previous bovine 50K genotyping.  The genotyping assays provided quantitative data that was used to assess chimerism in blood-derived DNA.  Monozygotic twins, as a percentage of all twin births, were 3.5 and 9.1% for the Jersey and Holstein breeds, respectively.  Jersey and Holstein breeds did not differ in proportion of nonchimeric twinsets at 20.1 and 15.7%, respectively ( > 0.05), providing no evidence for genetic variation in anastomosis.  The degree of chimerism for members of a twinset was also evaluated with regard to representation of self vs. co-twin in the blood-derived DNA.  For twinsets where the more chimeric twin was 45% or greater co-twin in its blood-derived DNA, there was a strong inverse relationship ( < 0.001) between percent co-twin in the blood-derived DNA of members of a twinset.  For twinsets where the more chimeric twin was less than 45% co-twin, there was no significant relationship between the degrees of chimerism in members of the twinset.  These results suggest that variation in chimerism in members of a twinset may be a function of degree of anastomosis and differences in timing of the migration of hematopoietic stem cells between members of the twinset.