Random match probability can be calculated in mixtures that involve two contributors and one contributor is known. For example, in sexual assault cases a vaginal swab is analyzed for the presence of semen and a DNA profile is developed from the sperm fraction and non-sperm fractions.

In some cases, as mentioned above, the sperm fraction results in a mixed sample that is consistent with the victim and the sperm donor. If the mixture reveals four alleles - 14, 15, 16, and 17 - and the victim is known to contribute the 14 and 15 alleles, it can be inferred that the sperm donor must be responsible for the 16 and 17 alleles.

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