These ones I always teach the interns at work here:

"How do you tell the sex of a chromosome? Unzip it's genes and take a close look."

"A student was idly reading the diplomas on his instructor's wall when he noticed that he had two PhDs, one in mathematics and an earlier one in cellular biology. Curious why he would make such a swing in commitments the student asked: 'Sir, why did you change from studying biology to studying math?' The professor said he liked to keep things simple and 'biology is the only science where division and multiplication happen at the same time!'"

"A scientist, fed up with looking at rocks all day, decided he was done with geology and went to work for CERN. Today he's the leading theorist on earthquarks."

"There was this biologist who was doing some experiments with frogs. He was measuring just how far frogs could jump. So he puts a frog on a line and says "Jump frog, jump!". The frog jumps 2 feet. Next he chops off one of the legs and repeats the experiment. "Jump frog jump!" he says. The frog manages to jump 1.5 feet. He chops off another and the frog only jumps 1 foot. He continues and removes yet another leg. " Jump frog jump!" and the frog somehow jumps a half of a foot. Finally he chops off the last leg. He puts the frog on the line and tells it to jump. The frog doesn't move.

While analyzing his data he comes up with:

Frogs with 4 legs can jump 2 feet.

Frogs with 3 legs can jump 1.5 feet.

Frogs with 2 legs can jump 1 foot.

Frogs with 1 leg can jump 0.5 feet.

Frogs with no legs are deaf."