Not Just Identical or Fraternal
Although appearance or mixed genders are often indicators of the DNA relationship between twins, our simple buccal (cheek) swab testing method decisively identifies whether twins are identical or fraternal. This can be important both from the point of view of mere curiosity, but also with respect to health considerations, and in the circumstance where twins or parents of twins may volunteer that they participate in twin studies, which typically require identical twins.
Zygosity is the measure used to determine the type of twins. Zygosity is the degree of similarity of alleles for a trait in an organism. Alleles are markers related to specific traits found in specific locations on a chromosome. We have historically understood that there were two types of twins, identical and fraternal. Identical twins form when a single egg is fertilized by a single sperm; within approximately 2-3 days after fertilization, the egg splits completely, resulting in two embryos with identical dna. Fraternal twins form when two separate eggs are released by the mother’s ovaries and fertilized by two separate sperm, resulting in two embryos with related but different dna.
Semi-Identical or Polar Body Twins
In March 2007, after speculation by scientists that identical and fraternal didn’t tell the whole story, a set of twins was born whose dna, when tested, showed that they were “semi-identical” twins, also known as “polar body twins.” It is theorized that semi-identical twins could result in two different possible ways. Both theories begin with with a single egg from the mother, with the result is twins that share identical maternal DNA, but only as much as half of the same paternal DNA.
In one possible scenario, the egg cell divides, without splitting, and then each cell is fertilized by a single sperm. The genes could have then been muddled up, mixed and matched so to speak, before the egg fully separated. But egg division before fertilization is thought to be very rare, so this view is the less-favored possibility.
The second possibility, which some scientists thinks is more likely to occur, is that a single egg was fertilized by two separate sperm. This would have created an embryo with three sets of chromosomes. The theory surmises that, at the embryo’s two-cell stage, one cell shed the chromosomes from one sperm, and the other cell the other sperm’s genes. The embryo would have subsequently split, but later in time than we typically expect in identical twins. It is theorized that hermaphrodite and conjoined twins are more likely to have resulted from this category of twinning.
Why Twin DNA Testing?
At the time of birth, the delivering physician can try and determine whether twins are identical or fraternal by the placenta, but this is not always clear and accurate. When twins are the same sex, the only definitive way to find out twin typing is through zygosity testing. Let ARCpoint Labs of Tempe help satisfy your curiosity.
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