In the case of imprinted genes, this means that the chromosomes would be homozygous for either the active or inactive allele. While the mechanisms producing such vanishing twins is not known, research has focused on vascular problems that result from identical twins sharing the same placental circulation. Fetal growth discordance is likely when one twin draws blood from a larger arterial zone in the placenta than the other. In other cases, however, growth discordance can be the result of blood flowing from one twin into the other. This can occur through the placental venous return regions or from anastomeses between blood vessels.
Polar body twins.
Between the concepts of monozygous twins and dizygous twins is the idea of polar body twins. In such cases, one sperm is thought to fertilize the egg, while another sperm is thought to fertilize a polar body. Such embryos without hearts are only found to exist in situations of twinning, where circulation from the normal twin keeps the abnormal twin alive. In this case, the abnormal, acardiac, twin was an XXX triploid, having three complete chromosome sets. Analysis of chromosomes and major histocompatability antigens showed that the acardiac twin was preoduced from the fertilization of a sperm with the diploid first meiotic division polar body.