Half siblings are 25% related by consanguinity as they share one parent and separated from each other by two generations (formula_3). There is a very small chance that two half-siblings might not share any genes, if they didn't inherit any of the same chromosomes from their shared parent. This is possible for full siblings as well, though even more unlikely. But because of how homologous chromosomes swap genes (due to chromosomal crossover during meiosis) during the development of an egg or sperm cell, however, the odds of this ever actually occurring are practically non-existent.