By Paige Freshwater More than 10 per cent of marriages worldwide are between first and second cousins \- but not everybody believes it’s a good idea. On average, a person shares around 12.5 per cent DNA with a first cousin, 6.25 per cent with a first cousin once remove and 3.13 per cent with a second cousin. However, one 21-year-old student claims 12.5 per cent was enough to leave him with physical and mental health defects due to his parents being first cousins. Venting on Reddit, he said: “My parents are cousins. They tried to obtain a marriage license in Texas. The state didn’t grant it bec…