Jax49
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Good question! Each person inherits approximately 1/2 of their dna from their father and approximately 1/2 from their mother. I'm just guessing here, but it might be possible (and likely rare) for two full siblings to match as identical twins if they both inherited the same exact dna from each parent. In a dna class I took, it was discussed there are of rare cases where two siblings do not show as sibling matches and the explanation is they each inherited opposite 1/2's of each parents' dna.
The odds against are apparently astronomical. bbm
Your DNA is arranged into chromosomes, which are grouped into 23 pairs. When a sperm cell is manufactured, the fathers genome is split in two, so that each sperm receives one chromosome from each of the 23 pairs, and the same thing happens with the egg cells in the mother. When the egg and the sperm combine to form an embryo, the resulting genome contains half the mothers chromosomes and half the fathers, chosen essentially at random.
Theoretically, same-sex siblings could be created with the same selection of chromosomes, but the odds of this happening would be one in 246 or about 70 trillion.
http://www.sciencefocus.com/qa/can-two-people-have-same-dna