Y-CHROMOSOME   :   

One of two sex chromosomes in higher organisms that defines the gender of the adult. In almost all sexually reproducing organisms, the Y-chromosome defines male characteristics.

YELLOW FEVER   :   

The transmission of acute viral disease by the bite of Aedes aegypti mosquito is known as yellow fever.

YOLK   :   

The part of an egg that helpsin feeding the embryo as it develops in an egg.