According to Jim Recap: A Hole in One
Jim and Cheryl have decided that they would like to have another child, but they just can't seem to make things work. This is compounded by the fact that Jim won't stop drinking, smoking and sitting in the hot tub all the time.
So when he comes home one night and Cheryl smells cigars, beer and polish sausage (with kraut) on his breath, she's had enough and she convinces him that if they are going to have a baby he needs to take things seriously. That works well until he hits a hole in one, at which point he drinks, smokes cigars and sits in the hot tub (sans suit) - all at the same time.
This doesn't go well for a number of reasons, but most notably because the next day is the big day at the fertility doctor, and Jim asks Andy for a big favor - the kind that you don't ask just anyone. Since Jim can't, well, perform, he asks Andy to take care of business and send in his sample instead. This actually works too well.
It works so well, in fact, that the fertility doctor (John Ducey, not the same doctor who will one day become Dana's husband) runs tests and wonders why it seems that Jim's and Cheryl's samples are so closely related. He wonders if they shared "a parent... or two".
Naturally, Cheryl and Dana figure out what happened, though not exactly why, and come up with a great scheme to get back at Jim and Andy by telling them that the sample was so good that Cheryl went ahead with the fertilization, thus making Jim and Andy think that they did a very bad thing indeed. It works, until Cheryl and Dana can't stand it anymore and let it slip that they knew all along, and that Cheryl is not, indeed, knocked up by her own brother's seed.
