Twinning in cattle is infrequent, and also undesired. It can result in increased occurrence of abortion and dystocia, reduced calf survival, and a high likelihood of freemartinism in mixed-sex twins. Twin gestations also commonly are associated with the formation of placental vascular anastomoses (PVA) between twins. Through this, they share blood, hormones (leading to freemartinism in mixed-sex twins) and hematopoietic stem cells, which are the progenitors of white blood cells. The sharing of stem cells between twins can result in leukochimeric twinsets; these are twins that have white blood cells derived from both self and co-twin owing to the fetal migration of hematopoietic stem cells from the extraembryonic mesoderm of the yolk sac to final sites like bone marrow and thymus. This study examined the degree to which this leukochimerism changes over time. DNA was extracted from hair bulbs to determine the individual's true genotype and blood samples obtained at six time points from 1 week to 8 months of age to assess leukochimerism. Samples were genotyped using a medium-density SNP chip, and quantitative estimates of allele frequency were determined using SNPs for which members of a twin set had alternative homozygous genotypes. Results indicated statistically significant changes in proportion self and co-twin with age and suggest that by four months of age, the genotypic mix in white blood cells represents the hematopoietic cell population resident in the individual. 

Genotype data were generated from hair-derived DNA samples and blood-derived DNA, the latter collected at 1 week, 1 month, 2 months, 4 months, 6 months and 8 months of age.

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