There aren't many common biology words that start with the letter "y". Here are a few:

* Yolk: The nutrient-rich, yellow part of an egg.

* Yeast: A single-celled fungus used in baking and brewing.

* Y chromosome: One of the two sex chromosomes in humans (the other being the X chromosome). Males have one X and one Y chromosome, while females have two X chromosomes.

* Yolk sac: A structure that develops in the embryo of animals and provides nourishment.

Let me know if you'd like to explore more specialized or uncommon terms!