Not Just Identical or Fraternal
Knowing where twins are fraternal, formed from two sperm fertilizing two eggs at the same time, or identical, formed from an egg splitting 2-3 days after fertilization can be important for satisfying curiosity, addressing health concerns and volunteering for identical twin studies. Although appearance or mixed genders are often used to make this assessment, zygosity is the only way to know for sure. Zygosity is the measure used to determine the type of twins and measures the degree of similarity of alleles, markers related to specific traits found in specific locations on a chromosome,for a trait in an organism. ARCpoint Labs of Austin, TX North can perform this test with samples taken via buccal (cheek) swab testing.
Semi-Identical or Polar Body Twins
In March 2007, set of twins was born whose DNA, when tested, showed that they were “semi-identical” twins, also known as “polar body twins.” This discovery came after speculation by scientists that identical and fraternal might not be the only categories necessary to accurately place twins. There are two theories as to how semi-identical twins could be formed, both beginning with a single egg. This results in shared DNA from the mother, but different DNA from the father.
In one possible scenario is less favorable because egg splitting before fertilization is thought to be very rare. In the situation, the egg cell divides without splitting. After that, both cells are fertilized by a single sperm. This creates the possibilities for the genes to “mix” before the egg fully splits.
The second possibility, which some scientists thinks is more likely to occur, is that an embryo with three sets of chromosomes splits at the two-cell stage. This would be the the result of two sperm fertilizing a single egg. The theory states that each cell shed the chromosomes from one sperm, leading to the split. The embryo would have subsequently split, but later in time than we typically expect in identical twins. Since this type of separation would occur later than for most other types of twins, it is theorized that hermaphrodite and conjoined twins are more likely to have resulted from this category of twinning.
Why Twin DNA Testing?
During the delivery process, a physician can try to guess the type of twins by the placenta, but a zygosity test is the only way to accurately twin type when babies are the same sex. If you are curious, let ARCpoint Labs of Austin, TX North help find out what type of twins are in your family.
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