Twins result from two distinct processes: the splitting of a single fertilized egg or the fertilization of two separate eggs.
Identical (Monozygotic) Twins
Identical twins originate from a single fertilized egg (zygote) that splits into two separate embryos. This splitting results in twins sharing the exact same genetic information. As noted by the NHS, "[Identical (monozygotic) twins happen when a single egg (zygote) is fertilised. The egg then divides in 2, creating identical twins who share the same genes.]" This process is not fully understood, but it's not typically considered genetically influenced. The Better Health Channel elaborates, stating that "[The self-contained halves then develop into two babies, with exactly the same genetic information.]"
Fraternal (Dizygotic) Twins
Fraternal twins, on the other hand, develop from two separate eggs fertilized by two separate sperm. These twins are genetically distinct, sharing only about 50% of their DNA, similar to any other siblings. This type of twinning is influenced by genetic factors, with a family history increasing the likelihood.
Conjoined Twins
A rarer occurrence is the development of conjoined twins. This happens when a single embryo partially separates, resulting in two individuals joined at some point on their bodies. The Mayo Clinic explains that "[Conjoined twins develop when an early embryo only partially separates to form two individuals. Although two babies develop from this embryo,...]" The exact cause of incomplete separation remains unclear.
Factors Influencing Twin Development
While the genetic predisposition plays a significant role in fraternal twinning, identical twinning seems largely unrelated to genetics. MedlinePlus notes that "[Research suggests that most cases of MZ twinning are not caused by genetic factors.]" Other factors like maternal age, ethnicity, and reproductive technologies may influence the chances of having twins.
In summary, twin development involves either the splitting of a single fertilized egg (identical twins) or the fertilization of two separate eggs (fraternal twins), with genetic factors primarily impacting the latter. The less common conjoined twinning results from incomplete embryo separation.
