When one pregnancy produces two babies, the result is twins. While born at the same time, twins inherit different traits and may not even look similar. Twins also tend to have different personalities and mannerisms in comparison to the other twin. The two most common types of twins are monozygotic and dizygotic. Monozygotic stands for identical twins who formed from one zygote, while dizygotic is fraternal twins who formed from two zygotes.
Even if born at the same time, twins may not be identical or share the same gender in a fraternal or sororal pairing, Sororal is for female fraternal twins. In the case of fraternal twins, the offspring are independently fertilized with two different sperm cells. Fraternal twins also have a very small occurrence of sharing the same chromosomes. They do share about 50 percent of their genes. Fraternal twins may even look identical but still did not share the same egg, therefore they are not classified as identical.
When one egg is fertilized but divides into two embryos, the result is a set of identical twins. These twins possess identical genes, and they are always the same sex. Even though identical twins share the same DNA, environmental factors and other influences change genetics over time. For example, when twins are adopted by different parents, they grow up differently and change slightly from one another. However, many twins also continue to be alike in IQ and personality throughout their lives.
A very small instance of semi-identical twinning occurs around the world. Semi-twinning occurs when twins take the same genes from the mother but different genes from the father. This often results from polar body twinning, when multiple sperm cells fertilize the ovum and second polar body. Semi-identical twins may look identical to one another or not. They also do not have to share the same gender. In one semi-identical case studied by Vivienne Souter, a geneticist from Banner Good Samaritan Medical Center, she found a semi-identical twin with both ovarian and testicular tissues.
Identical twins who join bodies in the womb are considered conjoined twins, also known as Siamese twins. Many times, one twin is stillborn in the pair. Scientists theorize that conjoined twins occur from fission, when the fertilized egg splits only partially. Another theory is fusion, where the eggs separate but similar stem cells in each twin find one another and fuse the twins together. Conjoined twins are classified into five common types according to which part of the body joins together. The most common type is thoraco-omphalopagus, when the twins form at upper chest to lower chest. These twins may share a heart, liver or another part of the digestive system.