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 characteristics may be different in personalities and mannerisms. 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.
Sororal or Fraternal Twins Facts
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, two eggs cells are independently fertilized with two different sperm cells. Fraternal twins share about 50 percent of their genes, the same percentage as other siblings. Fraternal twins may even look alike but still did not share the same egg, therefore they are not classified as identical. Boy girl twins occur in fraternal twins. Fraternal twin personality traits differ in the same way as other non-twin siblings, especially when the twins are treated as individuals rather than as a twin set.
Identical Twins Facts
When one egg is fertilized but at the two cell stage 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 non-shared experiences cause differences in personality over time. For example, when twins are adopted by different parents, they grow up differently and change slightly from one another. However, many identical twins also continue to be alike in IQ and personality throughout their lives.
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When the fertilized cell divides at about five days after fertilization, mirror image twins develop. These genetically identical twins may exhibit opposite features, such as mirror image fingerprints. Rarely, one twin's internal organs may be on the opposite side from the usual placement.
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 share body structures are considered conjoined twins, formerly referred to as Siamese twins. Many times, one twin is stillborn in the pair. Scientists hypothesize that conjoined twins occur from fission, when the fertilized egg splits only partially. Another hypothesis 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.