Twinning, the biological process of a single pregnancy resulting in two offspring, occurs in two fundamentally different ways that determine the twins’ genetic relationship and physical development. The scientific terms for these two types are monozygotic and dizygotic, commonly known as identical and fraternal twins, respectively. Understanding the differences between these two methods of conception provides a clear foundation for grasping the lifelong physical and genetic distinctions between the twins.
The Biology of Identical Twins
Identical twins, or monozygotic twins, begin their development when a single sperm fertilizes a single egg, forming one zygote. This single fertilized egg then unexpectedly splits into two separate embryos very early in the developmental process. The timing of this split is the significant factor that dictates the structure of the twins’ support systems within the uterus, specifically how they share the placenta and amniotic sacs.
If the split occurs within the first three days following fertilization, each twin develops its own chorion and amnion, resulting in two separate placentas and two separate amniotic sacs, known as dichorionic-diamniotic twins. This arrangement is present in about one-third of identical twin pregnancies.
However, the majority of identical twins split between days four and eight of development, after the chorion has already formed. In this scenario, the twins have separate amniotic sacs but share a single placenta and outer membrane, a condition called monochorionic-diamniotic. In rare cases, the division happens even later, leading to monochorionic-monoamniotic twins, where they share both a single placenta and a single amniotic sac. All monozygotic twins originate from the same single fertilized egg, meaning they possess virtually identical genetic material.
The Biology of Fraternal Twins
Fraternal twins, scientifically termed dizygotic twins, result from a completely different and more common process of conception. This type of twinning occurs when the mother releases two separate eggs during a single menstrual cycle, a phenomenon known as hyperovulation. Both of these eggs are then fertilized by two different, individual sperm cells.
Because two entirely separate fertilization events take place, two distinct zygotes are formed simultaneously. These twins are genetically no more similar than any other siblings born years apart, sharing an average of about 50% of their DNA. Each embryo implants separately in the uterine wall, developing its own placenta, chorion, and amniotic sac, a dichorionic-diamniotic arrangement. This process is often influenced by factors like genetics, maternal age, and fertility treatments.
Comparing the Genetic Outcomes
The fundamental difference in their biological origin results in profound distinctions in the genetic and physical outcomes for identical and fraternal twins. Identical twins share nearly 100% of their DNA, meaning they are genetically predisposed to have the same hair color, eye color, and physical features. Fraternal twins, conversely, share only about 50% of their genes, which is the same genetic overlap as any two non-twin siblings.
A straightforward way to distinguish between the two is by their gender possibilities. Since identical twins develop from one zygote containing a single set of sex chromosomes, they must always be the same sex, either both male or both female. Fraternal twins, originating from two separate eggs and two separate sperm, can be the same sex or different sexes, just like any other pair of siblings.
The structures supporting the twins during pregnancy also offer a contrast. Fraternal twins always have separate placentas and sacs, making them dichorionic and diamniotic. Identical twins, while always sharing the same genetic blueprint, can have varying placental arrangements depending on the timing of the split. Early-splitting identical twins will also be dichorionic-diamniotic, sharing no more placental structures than fraternal twins. However, the majority of identical twins are monochorionic, sharing one placenta, which is a key clinical differentiator.