DNA Persists in Mouth after Kiss
So romantic! ... when you kiss your partner passionately, not only do you exchange bacteria and mucus, you also impart some of your genetic code. No matter how fleeting the encounter, the DNA will hang around in their mouth for at least an hour. This means that women's saliva could contain evidence of unwanted attention in cases of assault, or even telltale signs of infidelity.
Natália Kamodyová and her colleagues at Comenius Univ. in Bratislava, Slovakia, recruited 12 couples who agreed to kiss each other passionately for at least 2 minutes. Afterwards, saliva samples were collected from the women at 5, 10, 30 and 60-minute intervals. Because Kamodyová's method relies on detection of the Y chromosome, it can only be used to identify a man's DNA in a woman's saliva.
Source: New Scientist