1. The passing of __________________ is the basis of heredity. 2. Our ________________ encode the instructions that define our traits. 3. Each of us has thousands of genes, which are made of _______ and reside in our chromosomes. 4. In addition to our genes, the _________________ we live in also helps define our traits. 5. Humans have two complete sets of _________ chromosomes. 6. When parents conceive a child, each parent contributes ____________ set of chromosomes. 7. Every child receives _______ of its chromosomes from the mother and half from the father. 8. This transfer takes place at ______________, when the father’s sperm joins the mother’s egg. 9. While most cells in our bodies have two sets of chromosomes, or a total of ______________, egg and sperm each have _________ chromosomes. 10. When egg and sperm unite they create a single cell called a __________________. 11. Each parent contributes _______ complete set of chromosomes to their child. 12. Since the parents contribute the chromosomes ______________ to each new child, every child inherits a unique set of chromosomes. 13. As a result, every baby will have a _______________ combination of traits.