It might seem obvious in most cases that two siblings are identical just by looking at them, but in cases where they have slight differences in appearances it may be necessary for a DNA test.
Identical or Fraternal
Identical or monozygotic twins will have an exact DNA profile. When the sperm met the egg, the zygote (baby to be) split and formed two embryos. Because the DNA contributed by the parents came from the same sperm and egg, the two children inherit the same DNA. With fraternal or dizygotic twins, the mother releases two or more eggs that can be fertilized by separate sperm giving them different DNA profiles. Fraternal twins are related in the same way as siblings. (see science of paternity)
What can a twin zygosity tell me?
An exact matching profile between two siblings is so unlikely to occur out of random chance that it can be concluded that an exact match confirms the identical twin relationship. In the event that the mother is unsure whether the two children are from the same father even though they were born together, we can perform a sibling test in conjunction with the twin zygosity at no extra charge.
