The staff at Jupiter Scientific presents a collection of biology jokes, though Daily Scan will let you decide which are genuinely humorous. The jeans/genes pun plays prominently on the list:
A: Pull down its genes.
A: Designer jeans.
For those who miss the puns' meanings, Jupiter Scientific has kindly provided explanations — though be warned that there is arguably nothing less funny than discussing the meaning behind an already unfunny joke.
HT: SciCareers.info