Asked by: Griffin Windheuser, by email
Your DNA is arranged into chromosomes, which are grouped into 23 pairs. When a sperm cell is manufactured, the father’s 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 mother’s chromosomes and half the father’s, 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. In fact, it’s even less likely than that. Before the chromosome pairs fully separate, they often swap individual genes from one chromosome to another in the pair. This means that even if successive sperm were manufactured with exactly the same chromosome selection, they wouldn’t contain the same genes.
Of course, a lot of the gene pairs in your genome are actually the same, so it wouldn’t matter which copy you used, but the odds of constructing an exact duplicate genome by chance are still vanishingly small. And even identical twins don’t necessarily have identical DNA. Stray cosmic rays and chemicals in our diet, such as caffeine and nicotine, can cause point mutations in random genes, here and there.