Identical twins share the same DNA profiles, which makes it difficult to use DNA evidence to narrow down which one was involved in a crime.

In some legal cases involving identical twins, actual fingerprint evidence can be used, because fingertip skin ridge patterns are unique even between identical twins. Human fingerprints are a quantitative trait and differ based on different prenatal environments in the womb. But what about DNA profiles of unrelated people? Scientists estimate that one human’s DNA profile differs from another’s by only 0.01%. Is it possible for two people to have the same DNA profile and could that result in a false conviction?

A person could be falsely convicted of a crime because his or her DNA profile happens to match that of the real perpetrator.

Questions

1. What nongene locations do scientists test for similar size when performing DNA profiling?
2. How many of these locations do scientists typically test?
3. For ease of calculation, let’s use one average instead of 13 different STR frequencies. Assume that each of the STRs used in profiling occurs 10% of the time.
What is the rough probability that two unrelated individuals would have the same 13 bands on a DNA profile?
4. There are approximately 7.4 billion people on Earth.
How does this number compare with the probability of a match you calculated before?
5. Reflect on your answers to questions 1–4. Explain why the statement bolded above sounds right, but isn’t.