A chromosome is a structure that is made of a chemical known as deoxyribonucleic acid, or DNA as well as protein. Chromosomes are found in the nucleus of cells. Chromosomes contain many genes. A gene is a molecular unit of heredity. Each gene codes for a polypeptide, which becomes a protein that performs a function in a living thing. The DNA molecules are very long and in order to fit inside the nucleus of a cell, they are highly coiled and condensed. During sexual reproduction, one haploid set of chromosomes is given to the offspring from each parent, forming a diploid amount of chromosomes in the offspring. Since chromosomes contain all of our genes, an organism that reproduces sexually gets a combination of genes from both parents. Genes that are expressed produce an organism's phenotype.