Who Fathered The Son?

A middle-aged couple with two beautiful daughters decided to try one last time for the son they always wanted. The wife became pregnant, and delivered a baby boy nine months later. The joyful father rushed to the nursery to see his new son, but was horrified to find an incredibly ugly baby. 'I cannot possibly be the father of that hideous child,' he said to his wife. 'Look at the two beautiful daughters I fathered.' When his wife blushed, he became suspicious. 'Have you been fooling around on me?' he demanded. His wife confessed: 'Not this time.'