Identical twins don't share exactly the same genetics, but the difference is so small that many DNA tests have trouble distinguishing one twin from another. It turns out the world's most sophisticated genetic tests have nothing on Czech police dogs.
There are roughly 11 million identical twins or triplets, and identical twins occur in about 3 out of every 1000 births. (The rate is substantially higher in pregnancies that are the result of in-vitro fertilization.) Identical siblings occur when a single egg is fertilized, but then splits into two separate embryos. As a result, identical twins will share exactly the same DNA at birth, although some environmental factors and random chance can cause certain genes to be expressed in slightly different ways. That said, studies show that certain traits of identical twins become more alike as they grow older, including intelligence and personality.
Whatever the slight variations between them, identical twins have similar enough DNA that it can be difficult to tell them apart using even the most cutting edge tests. But scientists recently demonstrated the amazing aptitude that police dogs have for telling the twins apart. Working with ten German shepherds serving in the Czech Republic's police force, researchers asked children to swab their cheeks and place the resultant DNA samples in glass jars.
Two of these children were identical twins, and another two were fraternal twins. The researchers then set up a fake crime scene investigation and provided the police dogs with the scent of one of the identical twins. The dogs were then asked to find that scent in one of the glass jars that had been placed in the room. Each of the ten dogs was given twelve trials, and every dog successful matched the right identical twin to the right jar in every single trial. They were similarly successful when asked to match other scents, including those of the fraternal twins.
This is a remarkable result, particularly because the identical twins involved in this study don't just share identical DNA - they also live in the same home and eat the same diet, leaving next to no possible environmental explanations for the dogs' success. It's an uncanny result, and it's perhaps the most amazing testament yet to the power of a dog's nose.