Rick stood there smiling, his throbbing cock just begging for release. Without saying a word, Karen darted over and swallowed his cock, before starting to slowly suck it.
While all this happened, Chad, now a full dog in both body and mind, just laid down and rested. He was all tuckered out from sex, and honestly would have been fine just being left alone.
Meanwhile, Rick was feeling the building pressure. He grabbed Karen's head and held her against his groin, so that when he blew his load, she was forced to ingest every drop. Once his dick began to soften, he let her go. Karen then fell over, gagging on the musky smell of his sperm.
"Good God, Rick, why does your cum taste so salty?"
"Simple. Because dog sperm is naturally higher in sodium than human."
It was then that Karen noticed the fur growing on her arms and legs. It was the same color as Chad's.
Rick sat down and stared right at her. Fur was also starting to grow on his limbs.
"Confused. That's understandable. See, all this has just been part of my experiment. Remember when I asked you to harvest Chad's sperm?"
"Yes, but--"
"Well, I already had the samples I needed. And then, while you and Chad were sticking it in, I put them into my body. When you swallowed my sperm, those same genes passed into you."
"But what's happening, Chad?!!"
"Don't you get it. You're becoming a dog. Well, actually, an anthropomorphic dog. Chad, you see, was just a prototype. Once I observed his transformation, I had all the information I needed to refine my work. Soon, you and I will be quite literally a pair of mutts."
"But why?"
"Because I'm want something more out of life. I want a family. And if there's one thing I've learned from animal science, it's that animals, especially those of the same species, have closer familial bonds than anyone."
By now, both he and Karen were fully covered with fur. Their hands and feet resembled paws, tails had grown out of their spines, and they both had muzzles, floppy ears, sharp teeth, and eyes with wider pupils.
"Rick, I have four fingers. And what happened to my nails?!!"
"Who the hell cares, Karen? You can either come and live with me in our new home, or you can stay and be captures and inevitably dissected."
"And Chad?"
"Almost forgot." He gave Chad a little prick behind the ears. In seconds, the formerly fully-grown dog boy was just a little puppy. Rick picked him up and placed him against Karen's chest, where a row of nipples had formed.
"You'll have to nurse him when we get to safety. After all, he's our son now."
"What--"
Before Karen could finish, Rick grabbed her paw and led her out through an open door. The sight of two naked dog-people standing upright and walking towards the woods got no attention from anyone at school. And as Rick predicted, Karen eventually settled into her role as a mother, nursing Chad, who she'd renamed Charles, until he was big enough to spend time with his father. And that was the last time anyone ever heard of them.