Mary-Kate and Ashley Olsen may look alike, but they are actually not identical (also called monozygotic) twins. The famous twins are fraternal (or dizygotic) twins. Contrary to common knowledge, similar appearance is not the sole basis to determine whether twins are monozygotic or dizygotic. Confused? This article will discuss the difference between monozygotic and dizygotic twins.
Summary Table
| Monozygotic Twins | Dizygotic Twins |
| Developed from a single egg which was fertilized by a single sperm cell | Developed from two eggs fertilized by two different sperm cells |
| Two fetuses grow in the same placenta | Two fetuses grow in two different membranes |
| Have almost identical genetic profile | Completely different genetic profile |
| Always of the same sex | May be of the same or opposite sex |
| May have the same physical and mental characteristics | May look alike or different; may behave similarly or differently |
| Also called “identical twins” | Also called “fraternal twins” |
Descriptions
Monozygotic twins, more commonly called “identical twins,” come from the same zygote (egg) fertilized by a single sperm. This fertilized egg then splits and produces two embryos which usually have identical chromosomes. Two fetuses will grow from the two embryos inside the same placenta.
Monozygotic twins have almost identical genes and the same blood type but they are still unique individuals. They are nearly always of the same sex although male-female monozygotic twins, an extremely rare occurrence, may result from a mutation during the early stages of development. Identical twins do not have the same fingerprints. They may look almost the same physically and may even share the same psychological and mental characteristics.
Dizygotic twins, also known as “fraternal twins,” are produced from two zygotes or fertilized eggs that are fertilized by two different sperm cells. In other words, dizygotic twins are somewhat similar to ordinary siblings except that the former are born at the same time. They do not share the same chromosomes.
Dizygotic twins are developed in two different amniotic membranes and placenta. They may share the same genes, but only 50% of them. They may resemble each other or may be physically different from each other. Additionally, dizygotic twins can be of the same or different sex.
Monozygotic vs Dizygotic Twins
What, then, is the difference between monozygotic and dizygotic twins?
Monozygotic twins, or “identical twins,” are developed from the same egg which is fertilized by a single sperm cell. Dizygotic twins, or “fraternal twins,” are developed from two eggs that are fertilized by two different sperm cells. Monozygotic twins are developed in one placenta whereas dizygotic twins are developed in two separate placentas.
Monozygotic twins have genetic profiles that are almost identical to each other. They have the same blood type and may even share the same personalities. Most of the time, they are also of the same sex and share the same physical features. On the other hand, dizygotic twins have completely different genetic profiles and are just like regular siblings. They may be of the same or different gender, and they may look alike or different.
